SYMPOSIUM Z



Diffusion Mechanisms in Crystalline Materials



April 13 - 16, 1998





Chairs

		 C.R.A. Catlow 		 Nick Cowern
		Royal Inst of Great Britain		Philips Research Laboratories
		 London, UNITED KINGDOM 		 Bldg WL-01
		44-171-4092992		Eindhoven,  NETHERLANDS
		 		 31-40-2742709
[3ex]		 Diana Farkas 		 Yuri Mishin
		Dept of MS&E		Dept of MS&E
		 Virginia Polytechnic Inst 		 Virginia Polytechnic Inst
		213 Holden Hall		213 Holden Hall
		 Blacksburg, VA 24061-0237 		 Blacksburg, VA 24061-0237
		540-231-4742		540-961-1526
[3ex]

Gero Vogl
Inst fur Materialphysik
Univ of Vienna
Wien, A-1090 AUSTRIA
43-1-313673204

Symposium Support
*Technology Modeling Associates, Inc.
*Virginia Polytechnic Institute

Proceedings published as Volume 527
of the Materials Research Society
Symposium Proceedings Series.

* Invited paper

SESSION Z1: DIFFUSION MECHANISMS IN METALS AND ALLOYS
Chairs: Yuri Mishin and Gero Vogl
Monday Morning, April 13, 1998
Golden Gate C1

8:30 AM *Z1.1
DIFFUSION IN METALS AND INTERMETALLIC COMPOUNDS. Helmut Mehrer, Institut für Metallforschung, Universität Münster, Münster, GERMANY.

Diffusion processes are fundamental and ubiquitous in the art and science dealing with solid materials at elevated temperatures. A knowledge of diffusion is therefore of interest for the production of materials and for their use in technological applications. In the present paper diffusion results from our laboratory in three related areas will be discussed: The first part is devoted to solute diffusion in Al and dilute Al-SiGe alloys. Contrary to most other metallic solvents, diffusion of transition elements in Al is anomalous in several respects: diffusion is very slow, activation enthalpies, pre-exponential factors and activation volumes are unusually high. By contrasst, non-transition elements in Al and Ge diffusion in Al-SiGe alloys show more or less normal solute diffusion behaviour. The solution seems to lie in a strong repulsive interaction between transition metals solutes and vacancies. The second part deals with intermetallic compounds. Particular attention will be devoted to recent investigations of Fe-Al and Fe-Si systems. Relevant factors for self-diffusion like the crystal structure, the state of order, the temperature and composition dependence are discussed also in connection with results from positron annihilation and Mö$\beta$bauer spectroscopy. Diffusion of selected foreign elements will be considered as well. An atomistic understanding of diffusion in intermetallics in terms of defect structure and diffusion mechanisms is obviously more complex than for metallic elements. There is, however, strong evidence that the defects which mediate diffusion are of vacancy type. In the final part diffusion studies on the quasicrystalline intermetallic compound Al-Pd-Mn will be presented. Diffusion of Mn, Fe and Zn has been studied in our laboratory as functions of temperature and hydrostatic pressure. In the temperature ranges investigated the quasicrystal diffusion is not significantly different from diffusion in related crystalline Al-base materials, for which a vacancy-type mechanism is generally accepted. Doc #20301

9:00 AM Z1.2
MOBILITY OF SELF-INTERSTITIALS IN FCC AND BCC METALS. Yuri Osetsky, Liverpool Univ, Dept. Mater. Sci. and Eng., UNITED KINGDOM; Anna Serra and Vicenc Priego, Univ. Politecnica de Catalunya, Dept. Matematica Aplicada III, Barcelona, SPAIN.

Diffusion of self-interstitial atoms has been studied by molecular dynamics using interatomic potentials of different types. In Fe, different potentials describe different stable configurations of the self-interstitial atom. Nevertheless, the temperature dependence of the diffusion mechanism was found to be qualitatively similar for all the potentials. At low temperatures, self-interstitials migrate one-dimensionally via <111> crowdion while at high temperatures a three-dimensional mechanism via <110> dumb-bell has been observed. In Cu all the potentials reproduce the same stable configuration which is <100> dumb-bell. The migration mechanism was found to be three-dimensional random walk via <100> dumb-bell for the long ranged pair potential. In the case of the short ranged many-body potential at low temperature, a significant contribution from a two-dimensional <100> dumb-bell has been observed. At high temperature another contribution from <110> crowdion was found. The methods to simulate and to treat results for the diffusion study have been discussed. The phenomenon of the interstitial interaction with the periodic boundaries was found. The method to avoid this interaction in the study of diffusion by molecular dynamics is suggested.

9:15 AM Z1.3
DIFFUSION OF BORON IN COPPER BY DIRECT-EXCHANGE MECHANISM. B. Ittermann, H. Ackermann, H.-J. Stöckmann, K.-H. Ergezinger, M. Heemeier, F. Kroll, F. Mai, K. Marbach, D. Peters, G. Sulzer, Fachbereich Physik der Universität Marburg, Marburg, GERMANY.

Using $\beta$-radiation detected nuclear magnetic resonance and cross-relaxation (CR) spectroscopy we studied lattice sites and diffusion behavior of implanted ¹²B impurities in single-crystalline Cu. Up to about 400 K we exclusively observe interstitial B (B_i) at octahedral sites migrating via a simple interstitial mechanism. Starting at 400 K part of the diffusing B_i encounter Cu vacancies from the own implantation damage and form substitutional B (B_s). At temperatures above 600 K also the B_s becomes mobile. From combined NMR and CR informations we conclude that no other lattice defect is involved in this B_s migration. This leaves only the spontaneous direct-site exchange between B_s and neighboring Cu atoms as the underlying diffusion mechanism. B_s in Cu is, to our knowledge, the first diffusion system where the long postulated direct exchange has ever been established experimentally.

9:30 AM Z1.4
LOCALIZED DIFFUSIONAL MOTION OF HYDROGEN AND DEUTERIUM IN CRYSTALLINE Pd₉Si₂. C. Karmonik¹, T.J. Udovic¹, Q. Huang¹, J.J. Rush¹, Y. Andersson², T.B. Flanagan³, ¹ NIST Center for Neutron Research, National Institute of Standards and Technology (NIST), Gaithersburg, MD; ² University of Uppsala, SWEDEN; ³University of Vermont, Burlington, VT.

Using a variety of neutron scattering techniques, we have investigated the dynamics of hydrogen and deuterium in solid solution with crystalline Pd₉Si₂. In powder diffraction experiments, we have identified two/three different interstitial sites for D/H absorption. The sites common for hydrogen and deuterium exhibit a temperature-dependent occupancy. The corresponding vibrational modes found in inelastic measurements suggest a different dynamic behaviour for D/H in these sites. Quasielastic scattering experiments have revealed that hydrogen and deuterium undergo a localized motion between two of the sites. Results are consistent with the presence of a temperature dependent, asymmetric double-well potential. The present study is an excellent example of the complementary nature of different neutron scattering techniques for elucidating hydrogen dynamics in solid-state materials. Further experiments with higher H/D concentrations are underway to explore the nature of these sites in more detail.

10:15 AM *Z1.5
DIFFUSION IN METALS AND INTERMETALLIC COMPOUNDS: THE IMPACT OF AB INITIO CALCULATIONS. Manfred Fähnle, Bernd Meyer, Jens Mayer, Juan S. Oehrens, Gabriel Bester, Max-Planck-Institut für Metallforschung, Stuttgart, GERMANY.

In ordered intermetallic compounds the mechanisms of self-diffusion are more complex than in monoatomic crystals: First, to maintain the homogeneity of the sample always various types of atomic defects must be generated simultaneously, and second, the diffusion processes must preserve the atomic order. Because experiments often yield only macroscopic information (concentrations, diffusion constants) on the collective behavior of all defects or limited microscopic information (e.g., on diffusion jump vectors rather than on diffusion paths), theory is required to determine the microscopic properties of the defects and thus to provide a basis for materials design. By a generalized grandcanonical statistical approach it is shown that the experimentally obtained effective defect formation parameters (formation energy, entropy and volume) depend in a generally complicated manner on suitably defined microscopic defect formation parameters of all possible atomic defects, and that this must be taken into account in order to avoid serious misinterpretations of experimental data. Ab initio electron theory is used to determine these microscopic parameters and to elucidate the electronic reasons for the different behavior of various materials. In a second step, possible diffusion paths are considered within the transition state theory by calculating the saddle point energies ab initio. The reliability of the ab initio electron theory is demonstrated for the elementary metals Li, Na, K, Al and Mo. Then the results on the formation and migration properties of atomic defects in B2-FeAl and in compounds with D0₃ structure (Fe₃Al, Fe₃Si, Ni₃Sb) are presented and discussed from the viewpoint of the electronic structure, and the impact on the interpretation of experimental data is outlined.

10:45 AM Z1.6
ANOMALOUS DIFFUSION IN BCC-TRANSITION METALS INVESTIGATED BY ELECTRONIC STRUCTURE CALCULATIONS. F. Willaime, A. Satta, O. Le Bacq, Section de Recherches de Metallurgie Physique, CEA Saclay, FRANCE.

The origin of the anomalous behaviour of the self-diffusion coefficient in BCC transition metals is investigated by performing systematic electronic-structure calculations of the vacancy parameters. The qualitative trends predicted from tight-binding models for the vacancy formation and migration energies as function of d-band filling are confirmed by quantitative Density Functional Theory studies along the 5d-series ($\beta$-Hf, Ta and W). These two approaches also underline the effect of finite electron-temperature, in particular in group-VI elements (Cr, Mo, W), and of structural relaxations, which are most important in group-IV elements ($\beta$-Ti, $\beta$-Zr, and $\beta$-Hf). The comparison with experiments support the mono-vacancy mechanism at low temperatures, with parameters having a strong dependence both on d-band filling and on temperature.

11:00 AM *Z1.7
FROM DIFFUSION MECHANISM TO CONFIGURATIONAL KINETICS. G. Martin, F. Soisson, M. Athenes, CEREM, Section de Recherches de Metallurgie Physique, CEA-Saclay, Gif sur Yvette, FRANCE; C. Desgranges, Framatome, Nuclear Fuel, Lyon, FRANCE.

Configurational evolutions in alloys, resulting from thermally activated vacancy jumps, are modelled by a Monte Carlo technique using a residence time algorithm with improved efficiency. Special attention is paid to the effect of the diffusion mechanism on the kinetic path of the configuration: interesting results are found in the early stages of unmixing as well as of phase separation coupled to ordering, in the BCC structure. The effect of the non conservation of vacancies is addressed in the simplest meanfield approximation of the above model: the formation and elimination of vacancies at discrete sinks distorts the interdiffusion profiles in a way which can be rationalised with simple arguments.

11:30 AM Z1.8
A NEW MICROSCOPIC KINETIC APPROACH TO CALCULATION OF THE PHENOMENOLOGICAL (ONSAGER) COEFFICIENTS IN ALLOYS. Maylise Nastar¹, Vladimir Yu. Dobretsov², Georges Martin¹,¹CEA,Gif-sur-Yvette, FRANCE; ²Russian Research Center, Kurchatov Institute, RUSSIA.

A new microscopic kinetic approach to calculation of the phenomenological coefficients for matter transport (sometimes called Onsager's coefficients) in alloys has been developed. This approach is based on the master equation which describes time evolution of the distribution function of a system. At equilibrium the distribution function is defined by the configurational Hamiltonian depending on physical interactions between atoms. To describe non-equilibrium states we use the same distribution function but with an effective Hamiltonian including time-dependent effective interactions completely determined from kinetic equations. The 0th-order approximation consists in taking into account only pairwise effective interactions. In this approximation the kinetic equations have been written and solved for a steady state closed to equilibrium. We give the microscopic analytical expressions of the diagonal and cross phenomenological coefficients for a homogeneous binary alloy where matter transport is controlled by the vacancy-mediated diffusion mechanism. These expressions only depend on the physical interactions, the alloy composition and the geometric correlation factor which reflects the crystallographic structure. Our approach allowed us to calculate this factor exactly. Comparison with values obtained from Monte Carlo simulations using Allnatt's time-correlation method shows quite good agreement: the relative difference is within the range 1 - 10 % both for positive and negative mixing energy except for the cross coefficients in a low concentrated alloy where it is about 20 %. To compare with the known phenomenological models we calculated within our approach the tracer diffusion coefficients and using, as an example, Manning's relations obtained new expressions for the phenomenological coefficients which turned to be different from our microscopic ones but quantitatively very closed (within 1%).

11:45 AM Z1.9
SIMULATION OF INTRINSIC DIFFUSION IN MULTICOMPONENT MULTIPHASE SYSTEMS. Martin Hunkel, Dietrich Bergner, TU BA Freiberg, Inst. of Physical Metallurgy, Freiberg, GERMANY.

The simulation of diffusion in multicomponent multiphase systems is of interest for many systems which are used in industrial applications. The use of intrinsic diffusion is a practicable way for the simulation of multicomponent multiphase systems. Especially if one can use Manning's random alloy model, intrinsic diffusion is an easy way to simulate diffusion effects. A simulation model for intrinsic diffusion is presented for multicomponent multiphase systems. The model is not restricted on a certain number of components or phases. For simplicity, Manning's random alloy model with vanishing vacancy wind effect is used for the model. Then the cross terms can be neglected. The simulation routine uses equations for the fluxes, the equation of continuity and an equation for the change of volume elements due to the vacancy flux. With this model diffusion paths, concentration profiles, fluxes of the components, vancancy fluxes as well as marker positions can be calculated. The shift of interfaces and the growth of new phases can also be determined. The simulation results were compared with experimental data of the Cu-Fe-Ni system. Diffusion was studied in single-phase areas and across an interface. The agreement between the simulated and the measured concentration profiles is good. The measured Kirkendall shift can be reproduced.

SESSION Z2: EFFECT OF PRESSURE AND STRESS ON DIFFUSION
Chair: Diana Farkas
Monday Afternoon, April 13, 1998
Golden Gate C1

1:30 PM *Z2.1
ADDRESSING ATOMISTIC MECHANISMS WITH HYDROSTATIC AND NONHYDROSTATIC STRESS. Michael J. Aziz, Division of Engineering and Applied Sciences, Harvard University, Cambridge, MA.

Measurements of the dependence of the diffusivity on hydrostatic pressure and nonhydrostatic stress provide direct information that cannot be obtained by other measurements about the operative atomistic mechanisms. A thermodynamic formalism has been developed for illuminating the predominant point defect mechanism of self and impurity diffusion and is used to provide a rigorous basis for point defect-based interpretation of diffusion experiments in biaxially strained epitaxial thin films in the Si-Ge system. Thermodynamics connect the effects of hydrostatic pressure and biaxial stress on diffusion with point defect-related dimension changes occurring on the atomistic scale. These dimension changes can now be predicted with molecular statics or dynamics simulations and compared directly with the measured stress effects. A combination of the hydrostatic and biaxial stress dependences of the diffusivity is either +1 or -1 times the atomic volume, depending upon whether the predominant mechanism involves vacancies or interstitials. Key experiments and simulation procedures for continued progress in elucidating point defect mechanisms will be reviewed and proposed.

2:00 PM Z2.2
THEORY OF DIFFUSION UNDER PRESSURE. Andrei V. Nazarov, E.A. Ivanova, Dept of Materials Science, Moscow State Engineering Physics Institute, Moscow, RUSSIA; Alexandr A. Mikheev, Dept. of Metal Physics, I.P.Bardin Central Res. Institute of Ferrous Metallurgy, Moscow, RUSSIA.

A new approach is suggested to resolve this problem. The influence of elastic stress on jump rate for atomic diffusion via the vacancy mechanism in crystals is evaluated by using statistical mechanics as was done by Glyde. As the stress fields can alter the surrounding atom configuration the height of the activation barrier is altered. The change of activation barrier is obtained to depend on the displacement field, symmetry of crystal, atom structure near the point defects and pair potential. Knowing this change it is possible to calculate the jump rate. The expression for the vacancies flow is obtained with the help of the hole gas method, by using jump rate [1]. Flow equations for binary system and interstitial solutes are obtained in a similar way. The diffusion processes under pressure are analyzed by using obtained equations. We consider some particular cases. In case of pure metals and binary alloys the expressions for migration and formation volumes are obtained and the calculation for some BCC and FCC metals is made at various temperatures. The activation volume of the interstitial solute diffusion is also calculated.
1. A.V.Nazarov and A.A. Miheev, Defect and Diffusion Forum, Vol 143-147, p177 (1997).

2:15 PM Z2.3
STRESS EFFECTS AND NON-LINEARITIES IN DIFFUSIONAL MIXING OF MULTILAYERS. Dezso Laszlo Beke, Peter Nemes, Zoltan Erdelyi, Istvan Andras Szabo, Gabor Langer, Department of Solid State Physics, L. Kossuth University, Debrecen, HUNGARY.

In the classical treatment of diffusional mixing of binary multilayers it is usually supposed that i) the intrinsic diffusion coefficients, Di, are independent of concentration (linear description) or have only a slight concentration dependence, and they are equal (the Kirkendall shift is zero) [1] ii) the stress effects are discussed in the continuum description can be analysed for a sinusoidal concentration distribution in the discrete Fick equations. In our lecture all of these limitations will be discussed. Numerical simulations are used to show the effects of the strong concentration dependence of Di. The effect of stresses are discussed in the framework of the treatment given by Stephenson [3], which contains the effect of simultaneous stress relaxation and contains the Kirkendall-shift as well. For the correct description of the Kirkendall-effect a generalised analytical expression for the gradient energy term is also given. The deviation from the continuum description is treated for the more realistic rectangular initial profile. The consequences of the above effects on the ln I/Io versus t profile (I is the intensity of the first small angle Bragg peak due to the concentration modulation) will be illustrated and changes in the critical wavelength - as compared to the linear treatment - will be also shown.
References
[1] Greer, A.L., Spaephen, F., in "Synthetic Modulated Structures" (eds. Chang, L.L. and Giessen, B.C.), Academic Press, New York, 1985, pp. 419-486
[2] Cahn, J.W. Acta Metall., 10, 179 (1962)
[3] Stephenson, G.B., Acta Metall., 36, 2663 (1988)

2:30 PM Z2.4
DISTRIBUTION FUNCTION FOR THE ACTIVATION VOLUME OF THE DISCONTINUOUS ORDERING REACTION IN THE Fe - 50 at. %Co ALLOY. D. Kolesnikov, M. Zenotchkin, J. Jun, W. Lojkowski, High Pressure Research Center, Warsaw, POLAND; V. Semenov, Institute of Solid State Physics, Chernogolovka, RUSSIA; E. Rabkin, TECHNION-Israel Institute of Technology, Haifa, ISRAEL; W. Gust, Institut für Metallkunde, University of Stuttgart, GERMANY.

We have investigated the effect of pressure on the kinetics of the discontinuous ordering (DO) reaction in the Fe -50 at.% Co alloy. During the DO reaction, the disordered matrix is replaced by an ordered structure and the ordering takes place by rearrangement of atoms in the migrating interface. The annealing temperature was 670 K. Annealing was carried out under gas pressures up to 1.2 GPa. For each pressure the distribution function for the rate of the DO reaction as a function of the grain boundary was determined. Based on the distribution functions for the reaction rate at each pressure, we determined the distribution function for the activation volume. It was found that the activation volume is in the range 0 - 0.25 of the average atomic volume of the alloy. It can be concluded that the DO reaction takes place at the grain boundary by a non-vacancy mechanism.

2:45 PM Z2.5
DIFFUSION OF STRAIN INDUCED DEFECTS AFTER HEAVY ION IRRADIATION. G. Aggarwal, P. Sen, School of Physical Sciences, Jawaharlal Nehru University, New Delhi, INDIA.

Ion irradiation is one of the many techniques employed for creating non-equilibrium concentration of defects. Diffusion of these defects control properties like energy of migration and greatly influence mechanical properties in a host metal. In polycrystalline metals, individual grains can be thought of as single crystals with dislocation lines providing natural interfaces. We have shown that ion irradiation causes strain across such interfaces [1]. Under high energy heavy ion irradiation defects other than point defects i.e. strain induced defects (SID) have been identified using scaling behaviour with temperature in Fe and Ni metal foils. Allowing non-equilibrium concentration of defects achieved during irradiation to come to equilibrium simulates annealing. In this process interaction of SIDs with dislocations have been established to follow $t^{\frac{1}{2}}$ law in these metals. Evidence of sessile ring formation due to defect migration has been established and residual resistivity is observed only in F.C.C. metals. The present understanding of the processes involved will be presented in some details.
Reference
1. P. Sen, G. Aggarwal and U. Tiwari, accepted in Phys. Rev. Letters.

SESSION Z3: DIFFUSION IN INTERMETALLIC COMPOUNDS - I
Chair: Yuri Mishin
Monday Afternoon, April 13, 1998
Golden Gate C1

3:30 PM *Z3.1
THEORY AND SIMULATION OF DIFFUSION KINETICS IN ORDERED ALLOYS AND INTERMETALLIC COMPOUNDS . Graeme E. Murch and Irina V. Belova, Department of Mechanical Engineering, the University of Newcastle, Callaghan, NSW, AUSTRALIA.

In this paper we review recent theoretical (simulation and analytical) advances made in understanding some of the complexities of self, impurity and chemical diffusion in ordered alloys and intermetallic compounds. The review is restricted to the B2, L1₂, A15 and DO₃ structures. Within the framework of a generalized vacancy mechanism some emphasis will be put on the means to assess the limits of usefulness of specialized mechanisms such as the six jump cycle, antistructural bridge and rotational antistructural bridge mechanisms.

4:00 PM Z3.2
CALCULATION OF THE DIFFUSIONAL PARAMETERS IN ORDERED Ni₃A1 ON A RELAXED LATTICE. Camilla Schmidt, Jean-Louis Bocquet, CEA, DECM, Saclay, FRANCE.

We use a semi-empirical N-body potential to calculate potential barrier heights and pre-exponential factors in an ordered Ni₃ Al alloy. These quantities enter the atomic jump frequencies which characterize the mass transport in the alloys. The crystal lattice is statically relaxed by applying a conjugate-gradient algorithm under constant pressure. The normal mode analysis is actually restricted to the calculation of the determinants of the force-constants matrices in the stable and saddle positions together with the search for the minimum eigen value at the saddle position. We focused on those few relevant mechanisms which are proposed to account for the experimental findings, namely the individual vacancy jump and the jump cycles. These quantities are evaluated in a perfectly ordered system as wall as in an alloy containing a nearby antisite defect. The tendency for the latter to lower the migration enthalpy of the vacancy gives some substance to a recent interpretation of the mass transport in terms of vacancy-antisite defect pairs.

4:15 PM Z3.3
ROLE OF A VACANCY-ATOM INTERACTION IN MONTE-CARLO SIMULATIONS OF LONG-RANGE ORDER KINETICS. Cristian Mocuta, Emmanuel Kentzinger, Véronique Pierron-Bohnes, and Marie-Claire Cadeville, IPCMS-GEMM, Strasbourg, FRANCE.

Monte-Carlo simulations of order-order kinetics in the ordered B2 structure of AB alloys have been performed in the frame of a Glauber formalism applied to a vacancy jump mechanism. Pair interaction energies including first and second nearest neighbours are taken into account, but we have restricted this study to the cases with: V_AA=V_BB=-V_AB. A saddle point energy can be added depending on the jumping atom. A constant vacancy concentration of 0.015% which is temperature independent is used in the simulations. Thus our model yields the migration part of the activation energy. An effective migration energy is deduced from the Arrhenius plots of the relaxation times of the order-order kinetics. When changing the ratio V²/V¹, this effective energy is found to vary linearly with the B2-A2 transition temperature which is deduced from the temperature variation of the long-range order parameter. Here the effect of an attractive interaction between the vacancies and the A atoms has been studied. We find that the predominant effect, mainly visible at low temperatures, is a preferential occupation of the B sites by the vacancies. The migration energy is slightly increased (5 to 10 percents). The B2-A2 transition temperatures are changed proportionnally to the mean interaction, averaged over all the pairs of sites in the sample. This study shows that the presence of such interaction could be partially at the origin of the asymmetries observed in some B2 aluminides: shape of the phase diagram around the stoichoimetry, different vacancy concentrations on both sublattices [1], and disymmetric variation of the activation energy for the interdiffusion with the concentration [2].
[1] FeAl: J.P. Rivière, Mat. Res. Bull. vol12, 995 (1977)
[2] NiAl: S. Shankar, L.L. Seigle, Metall. Trans. vol9A, 1467 (1978).

4:30 PM Z3.4
MECHANISMS OF Al SELF- AND Al-SUBSTITUTING SOLUTE DIFFUSION IN Ni₃Al. Sergiy Divinski*, Stefan Frank, Christian Herzig, Institut für Metallforschung, Westfalische-Wilhelms Universität, Münster, GERMANY. *Permanent address: Institute for Metal Physics, National Academy of Sciences of Ukraine, Kyiv, UKRAINE.

Experimental investigations of Al self-diffusion in Ni₃Al are to some extent restricted by a lack of relevant tracers. The temperature dependencies of the Al substituting solute diffusion of Ge, Ga, Nb, and Ti in Ni₃Al (75.9 at.% Ni singe crystals) were therefore measured in our group. A theoretical approach is derived to explain the features of both Al self-diffusion and Al-substituting solute diffusion in Ni₃Al. The model relies on the diffusion of Al anti-structure atoms by so-called anti-structure bridges. The approach incorporates not only the nearest-neighboring (nn) jumps of Al anti-structure atoms via the Ni sublattice, but also the two-step jumps at positions of 2nd, 3rd and 4th coordination shells. This drastically increases the Al diffusivity in comparison with only nn jumps. Although diffusion of Al and Ni atoms invokes the same vacancies on the Ni sublattice, Al atoms can diffuse even faster than Ni atoms at high temperatures. The contribution of the anti-structure bridge mechanism is shown to be different for self- and impurity diffusion and depends crucially on the anti-structure defect formation energy. The model allows to explain the observed diffusion behaviour of the Al-substituting solutes.

4:45 PM Z3.5
X-RAY DIFFUSE SCATTERING DATA IN THE DETERMINATION OF THE THERMODYNAMIC FACTOR IN INTERDIFFUSION FOR B2 FeAl-PHASE. Simon Dorfman, Dept. of Physics, Israel Institute of Technology-Technion, Haifa, ISRAEL; Vlad Liubich, David Fulks, Mater. Eng. Dept., Ben-Gurion University of the Negev, Beer Sheva, ISRAEL; Helmut Mehrer, Inst für Metallforschung, Universität Münster, Münster, GERMANY.

The interdiffusion coefficient is related via the modified Darken equation to the tracer diffusivities of the constituents. The thermodynamic factor $\Phi$ entering the Darken equation for interdiffusion coefficient is expressed in terms of the energetic parameters which describe the ordering process in the binary alloy. Our calculations show the dependence of $\Phi$ on the long-range order parameter. These calculations are performed for B2-phase of FeAl alloy with the energy parameters extracted from the X-ray diffuse
scattering data.

SESSION Z4: DIFFUSION IN INTERMETALLIC COMPOUNDS - II
Chairs: Helmut Mehrer and Hans-Eckhardt Schaefer
Tuesday Morning, April 14, 1998
Golden Gate C1

8:30 AM *Z4.1
ATOMIC DEFECTS IN INTERMETALLIC COMPOUNDS AND DIFFUSION PROCESSES. Hans-Eckhardt Schaefer and Roland Würschum, Universität Stuttgart, Institut f. Theoret. u. Angewandte Physik, Stuttgart, GERMANY.

The high-temperature features of the technologically relevant binary transition metal aluminides are governed by atomic defects in thermal equilibrium. Substantial progress has been achieved recently by specific studies of the formation [1] and migration [2] of thermal vacancies by means of positron lifetime spectroscopy and concomitant theoretical efforts. This contributes decisively to the understanding of the wide variation of the self-diffusivities observed in intermetallic compounds. In addition the defect properties may be closely linked to the high-temperature mechanical properties as demonstrated for the yield stress anomaly in B2-FeAl [3]. Initial studies of thermal vacancy formation in nanocrystalline Fe₃Si intermetallics will be discussed [4].
[1] H.-E. Schaefer and K. Badura, Def. Diff. Forum 143-147, 193 (1997)
[2] R. Wurschum et al. PRL 75, 97 (1995)
[3] H.-E. Schaefer et al., phys.stat.sol. (a) 160, 531 (1997)
[4] R. Wurschum et al., PRL, to be published.

9:00 AM Z4.2
POINT DEFECTS AND THEIR BEHAVIOR IN IRON ALUMNIDES. Joachim Wolff, Matthias Franz, Andree Broska, Bernd Kohler, Theodor Hehenkamp, Georg-August Universität, Institut für Metallphysik, Göttingen, GERMANY.

Ordered FeAl alloys as high-temperature structural materials have received considerable attention during the last few years. The applications and properties may be strongly composition-dependent as well as temperature-dependent due to the different types of point defects and their concentrations. Doppler broadening and positron lifetime measurements were carried out in ordered FeAl alloys from room temperature to 1500 K as well as with hydrostatic gas pressure up to 550 MPa. Additional differential dilatometric measurements as well as diffusion and isotope effect experiments were performed to clarify the nature of the atomic defects in iron aluminides. In the FeAl system the vacancy formation enthalpy decreases in the unordered A2 phase. In the range of the B2 phase the formation enthalpy is found to be nearly constant at 1 eV. Additionally, defect concentration from differential dilatometric measurements up to 3x10^-2 are detected. The formation volume of the defects can be determined from measurements varying the pressure at constant temperature. The formation volume increases from 0.88 $\Omega$ in Fe7at%Al to 1.4 $\Omega$ in Fe40at%Al ($\Omega$ = mean atomic volume). In quenching experiments the migration enthalpy is measured using the fast Doppler broadening technique. Two different migration enthalpies are observed. One low enthalpy of nearly 0.5 eV and a very high enthalpy of 1.6 eV comparable to the high activation energies in the order of 3 eV derived from diffusion experiments in the B2 phase. The present results suggest that thermal defects in the B2 phase are mainly triple defects at low temperature and double vacancies at higher temperatures. In the A2 as well as in the D0₃ structure single vacancies are supposed to be the predominant defect types. In consequence two diffierent diffusion mechanisms should result as limiting cases.

9:15 AM Z4.3
POINT DEFECTS AND SELF-DIFFUSION IN NICKEL-BASED L1₂ ORDERED COMPOUNDS. Teruyuki Ikeda, Taku Korata, Nobuaki Kurita, Hiroshi Numakura, Masahiro Koiwa, Kyoto Univ, Dept of Mater Sci Eng, Kyoto, JAPAN; Abderrahim Almazouzi, Paul-Scherrer-Institut, CRPP, Villigen, SWITZERLAND; Katsuhiko Nonaka, Iwate Univ, Dept of Mater Sci Tech, Morioka, JAPAN; Wolfgang Sprengel, Hideo Nakajima, Osaka Univ, ISIR, Ibaraki, JAPAN.

The mechanism of self-diffusion in Ni₃Al, Ni₃Ga and N₃Ge has been studied by tracer diffusion experiments, single-phase interdiffusion experiments and molecular-statics simulation using the embedded atom method. The diffusion coefficient of Ni has turned out to be approximately equal for the three compounds when normalized to the melting temperature of each material, whereas the diffusivity of the minor element is widely different: Ga > Al $\gg$ Ge. The interdiffusion coefficient increases with increasing the concentration of the minor element in all the three compounds, the trend being most remarkable in Ni₃Ge. These features can be accounted for by the $\alpha$-sublattice vacancy mechanism, in which the diffusion of both atom species is assumed to occur predominantly via the ordinary vacancy mechanism in the sublattice of the major element. With this model for self-diffusion, the molecular-statics simulation gives a reasonable explanation also for the mechanical relaxation effect observed in Ni₃Al: since the symmetry of an $\alpha$ sublattice site is tetragonal, the effect can be attributed to stress-induced reorientation jumps of antisite Al atoms between $\alpha$-sublattice sites.

9:30 AM Z4.4
BULK SELF-DIFFUSION IN B2 ORDERED NiAl-TEMPERATURE AND COMPOSITION DEPENDENCE. Stefan Frank, Christian Herzig, Institut für Metallforschung, Westfalische Wilhelms Universität, Münster, GERMANY; Ulf Södervall, Department of Physics, Chalmers University of Technology, Göteborg, SWEDEN.

In ordered alloys random vacancy motion is not possible as it could disrupt the equilibrium ordered arrangement of atoms on lattice sites. If the ordered lattice is to be retained, it is necessary to invoke more complex diffusion mechanisms considering the crystal- and defect structure and the effect of temperature and composition. NiAl exhibits a variable defect structure, where excess Ni atoms occupy Al lattice sites in Ni-rich NiAl (antistructure atoms) and constitutional Ni vacancies form in Al-rich compositions. Several possible diffusion mechanisms in ordered B2 compounds have been discussed, but neither experimental nor theoretical considerations provide an unambiguous basis for specification of the mechanism in NiAl at the present time. Bulk Ni self-diffusion in well defined NiAl single crystals was investigated experimentally for a set of compositions in a wide temperature range. Two different techniques were applied to determine the diffusion coefficients, namely tracer experiments by conventional serial sectioning using the ⁶³Ni radiotracer and SIMS analysis using highly enriched ⁶⁴Ni. Particular attention was devoted to the influence of thermal and constitutional vacancies on bulk diffusion at lower temperatures (below 1300 K). The effect of composition, i.e. constitutional defects, on diffusion behaviour in NiAl alloys was systematically investigated. New experimental results are presented and discussed in terms of current diffusion mechanisms.

9:45 AM Z4.5
MEASUREMENTS OF THE DIFFUSION OF IRON AND CARBON IN SINGLE CRYSTAL NiAl USING ION IMPLANTATION AND SECONDARY ION MASS SPECTROMETRY. R.J. Hanrahan Jr., Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM; S.P. Withrow, Oak Ridge National Laboratory, Solid State Division, Oak Ridge, TN; and M. Puga-Lambers, Microfabritech, University of Florida, Gainesville, FL.

Classical diffusion measurements in intermetallic compounds are often complicated by low diffusivities or low solubilities of the elements of interest. Using secondary ion mass spectrometry for measurements over a relatively shallow spatial range may be used to solve the problem of low diffusivity. In order to simultaneously obtain measurements on important impurity elements with low solubilities we have used ion implantation to supersaturate a narrow layer near the surface. Single crystal NiAl was implanted with either ⁵⁶Fe, or ¹²C in order to investigate the measurement of substitutional (Fe) versus interstitial (C) tracer diffusion using this technique. Specimens were annealed at temperatures ranging from 600 to 850&#186;C for periods of up to 3 hours. Depth profiling was performed using positive dynamic SIMS. Depth profiles were calibrated on each sample by sputtering several locations for periods of 5 to 60 minutes and measuring the resulting craters using profilometry. Possible inhomogeneities in local topography were investigated using optical and scanning electron microscopy. Diffusion coefficients were extracted from analysis of the depth profiles beyond the range of the original implanted ions.

10:30 AM *Z4.6
DETERMINATION OF THE DIFFUSION MECHANISM BY A METHOD WITH NEW POSSIBILITIES: NUCLEAR SCATTERING OF SYNCHROTRON RADIATION. Bogdan Sepiol, Institut f. Materialphysik d. Univ. Wien, AUSTRIA.

Nuclear scattering of synchrotron radiation (SR) is a new method for probing the elementary diffusion jump in crystalline lattices on an atomistic scale. We demonstrate its potential by a study of Fe diffusion in two intermetallic alloys: Fe₃Si (D0₃ structure) [1] and FeAl (B2 structure). As its sister methods, quasielastic M{ossbauer spectroscopy and quasielastic neutron scattering [2], nuclear scattering of SR provides information on rate, direction and distance of elementary jumps, but offers considerable advantages: While the high brilliance of SR permits measurements on tiny single crystals, the high intensity of SR will allow the investigation of unstable or metastable systems through fast measurements and in the future enable studies on other than Mössbauer isotopes by the use of Rayleigh scattering.
[1] B. Sepiol et al., Phys. Rev. Lett. 76, 3220 (1996).
[2] B. Sepiol, Defect and Diffusion Forum 125-126, 1 (1995);G. Vogl, Physica B226, 135 (1996).

11:00 AM Z4.7
MICROSCOPIC DIFFUSION MECHANISM OF IRON IN FeAl REVISITED BY A NEW METHOD. Bogdan Sepiol, Gero Vogl, Inst. f. Materialphysik d. Univ. Wien, AUSTRIA; Christopher Czihak, Univ. Wien and ILL Grenoble; Joachim Metge and Rudolf Rüffer, ESRF Grenoble, FRANCE.

The elementary diffusion mechanism of atoms in ordered stoichiometric B2 structure alloys is of special interest in diffusion studies. There is extensive dispute on its nature in this simplest structure of all intermetallic alloys. Since in the B2 alloys all nearest-neighbor sites are occupied by the atoms of the other species, only non-standard microscopic diffusion mechanisms can explain atomic movement. The results of conventional tracer (macroscopic) technique provide only information averaged over time and space and conclusions concerning the elementary diffusion jump are indirect. The elementary diffusion jump in crystalline solids can be determined by methods derived from nuclear physics. These are quasielastic M{ossbauer spectroscopy (QMS), quasielastic neutron scattering and, recently nuclear forward scattering of synchrotron radiation. Earlier we have reported on a determination of the elementary iron diffusion jump in stoichiometric FeAl by QMS [1]. We have found jumps between lattice sites in [100] and [110] directions. Moreover, the more distant jumps were preferred. Our interpretation was, that jumps are performed via a short-time intermediate residence on antistructure sites on the aluminum sublattice. Until now first-principle calculations, however, have not been able to find a way to reproduce this jump mechanism [2]. Therefore it appeared important to check the earlier result with nuclear forward scattering of synchrotron radiation, the method providing higher accuracy than QMS. We shall report on the result. [1] G. Vogl and B. Sepiol, Acta Metall. Mater. 42, 3175 (1994). [2] J. Meyer and M. Fahnle, Defect and Diffusion Forum 143-147, 285 (1997).

11:15 AM Z4.8
VACANCY MOBILITY IN NICKEL ALUMINIDE AS A FUNCTION OF COMPOSITION. Bin Bai and Gary S. Collins, Dept of Physics, Washington State Univ, Pullman, WA.

Using perturbed angular correlation of gamma rays (PAC), point defects in the B2 phase of NiAl have been detected by electric-field gradients they produce at nearby In/Cd probe atoms. Observed signals have been attributed to one or more Ni-vacancies in the near-neighbor (nn) shell. For Ni_1+2xAl_1-2x, measurements were made of site-fractions of probes with nn vacancies over the composition range 50 to 54 at. percent Ni (x= 0 to 0.04) after rapid quenching and at high temperature. Site-fractions were analyzed to obtain the vacancy concentration, [V], using a thermodynamic model that accounts for probe-vacancy binding (measured separately) and assuming that the formation enthalpy of the equilibrium defect is independent of temperature. The concentration measured at 1460 K was found to vary in good approximation as x^-1/2, in agreement with the expectation that the equilibrium defect is the triple-defect (two Ni-vacancies and one Ni-antisite atom). Due to trapping of vacancies at probe atoms during quenching, site fractions quenched-in are 5 times greater at stoichiometry (x= 0) and 10 times greater at x= 0.04. The composition dependence of the enhancement in site-fractions due to quenching is explained quantitatively in terms of the diffusivity D of Ni in NiAl, measured by Hancock and McDonnell to increase rapidly with x, D$\approx$ D₀exp(50x). Thus, the vacancy mobility , D/[V], increases very strongly with x in spite of a decreasing vacancy concentration. A detailed analysis of the mobility of Ni-vacancies will be presented that considers two diffusion mechanisms: second-neighbor jumps (on the Ni-sublattice) and jumps by the anti-structure bridge mechanism, in which Ni-vacancies make first-neighbor jumps much more readily through sites containing Ni-antisite atoms, the number of which increases with x.
This work was supported in part by the National Science Foundation under grant DMR 96-12306.

11:30 AM Z4.9
MECHANISM OF ``ORDER-ORDER'' KINETICS IN L1₂ SUPERSTRUCTURE STUDIED BY COMPUTER SIMULATION. Piotr Oramus, Rafal Kozubski, Institute of Physics, Jagellonian University, Kraków, POLAND; Marie-Claire Cadeville, Veronique Pierron-Bohnes, GEMME, I.P.C.M.S., Strasbourg, FRANCE; Wolfgang Pfeiler, Institut für Materialphysik, University of Vienna, Wien, AUSTRIA.

Monte Carlo simulations of the isothermal long-range order (LRO) relaxation in A₃B system with L1₂ superstructure have been performed within a model based on a vacancy jump mechanism between nearest neighbour sites. The studies aimed at the explanation of the origin of ``double'' LRO relaxations experimentally observed in Ni<sub>3</sub>Al: - i.e. an appearance of an initial fast relaxation process running in parallel with the slow one. An effect of pair interaction potentials, activation barriers for the migration of A- or B-atoms and of the vacancy concentration was studied. The values of pair interaction potentials controlled the vacancy site occupation preference, as well as the character of the ``order-order'' relaxations. The preference for vacancy residence on the A-sublattice (face centres), commonly postulated for Ni₃Al, occurred only for a narrow range of the pair-interaction potentials within the domain corresponding to L1₂ ordering. It was definitely indicated that the appearance of the fast relaxation process requires that the B-atom jumps predominate the A-atom ones within the initial stage of the relaxation. In addition, the effect was specifically correlated with the values of the activation barriers for atomic migration: the fast disordering relaxation was damped when high activation barriers were assigned to the majority A-atoms. The above results lead to a microscopic model of the ``order-order'' relaxation in L1₂ superstructure and yield new criteria for the evaluation of interatomic potentials in the crystals of L1₂ intermetallic compounds.

11:45 AM Z4.10
KINETICS AND MECHANISMS OF INTERMETALLIC GROWTH BY BULK INTERDIFFUSION. L.N. Paritskaya, V.V. Bogdonov, Department of Physics of Crystals, Kharkov State University, Kharkov, UKRAINE; Yu. S. Kaganovskii, Department of Physics, Bar-Ilan University, Ramat-Gan, ISRAEL.

The principal feature of intermetallic compounds growing during interdiffusion between two metals is the considerable deviations of their local compositions from stoichiometry. The wider compound area on the phase diagram, the greater this difference. As a sequence, unconventional diffusion mechanisms and accompanying phenomena may arise from existence of non-stoichiometric vacancies and their directed fluxes during reaction diffusion. Diffusion couples of multiphase Ni-Cd and Cu-Zn systems with wide $\gamma$-phase concentration interval (.10 at %) have been used to study the formation and growth of $\gamma$-phases Ni₅Cd₂₁ and Cu₅Zn₈ by means of optical microscopy, SEM and electron probe microanalysis under the conditions when only one $\gamma$-phase has been formed. The measurments of Kirkendall effect using inert marks have shown unipolar growth of Ni₅Cd₂₁ on the interface with Ni and of Cu₅Zn₈ on the interface with Cu. In Cu-Zn system $\gamma$-phase grows according to parabolic law showing diffusion limited growth. In Ni-Cd system $\gamma$-phase grows in diffusional-kinetic regime indicating the growth governed by both the kinetics of interfacial reaction and the diffusion. On the basis of obtained Arrhenius equations for both interfacial reaction and diffusion the new diffusion mechanisms in non-stoichiometric intermetallics have been proposed and discussed.

SESSION Z5: DIFFUSION ALONG GRAIN BOUNDARIES AND DISLOCATIONS -
DIFFUSION IN QUASICRYSTALS.
Chairs: Diana Farkas and Yuri Mishin
Tuesday Afternoon, April 14, 1998
Golden Gate C1

1:30 PM *Z5.1
GRAIN BOUNDARY DIFFUSION AND SOLUTE SEGREGATION IN POLYCRYSTALS AND ORIENTED BICRYSTALS. Christian Herzig, Thomas Surholt, Institut für Metallforschung, Universität Münster, Münster, GERMANY.

Recent progress in grain boundary (gb) diffusion research provides a deeper understanding of gb properties in relation to atomic mobility, solute segregation, gb structure and gb chemistry. In particular, it was possible by carefully designed radiotracer experiments to separately determine solute diffusion and solute segregation in thermodynamic equilibrium conditions by gb diffusion experiments in different kinetic regimes. The investigation of differently behaving solute - solvent systems on Cu and Ag basis (Se/Cu; Au/Cu; Se/Ag; Te/Ag; Ni/Ag) yield information on solute gb diffusion and segregation in relation to solute - solvent atomic binding and solute - vacancy interaction in the bulk and in gbs. Self- and solute gb diffusion experiments (⁶⁴Cu, ¹⁹⁵Au in Au; ⁷¹Ge in Al) in oriented and precisely characterized bicrystals in dependence on orientation and temperature clearly reveal a minimum in gb-diffusion and a maximum in the activation enthalpy at special $\Sigma$ value orientations. It is demonstrated that the additional dislocation network introduced in the gb plane by deviations from the exact $\Sigma$ value influences the diffusivity. Obviously, this influence depends strongly on the gb characteristics, e.g. $\Sigma$=5, $\Theta$=36.9&#186; (310) [001] and $\Sigma$=7, $\Theta$=38.2&#186; (12-3) [111].

2:00 PM Z5.2
EFFECT OF ATOMIC ORDERING ON IRON AND COBALT GRAIN BOUNDARY DIFFUSION IN THE FeCo EQUIATOMIC COMPOUND. Zsolt Tokei, Jean Bernardini, Laboratoire de Metallurgie, UMR 6518 CNRS/Aix-Marseille III, Faculte St. Jérôme, FRANCE; Dezso Beke, Department of Solid State Physics, L. Kossuth University, Debrecen, HUNGARY.

The few volume diffusion studies in intermetallic compounds that present an order-disorder transition have already been shown that the atomic ordering has an influence on material transport [1]. However, up to now the effect of atomic ordering on grain boundary diffusion has never been considered. Here we present a first study of the influence of order-disorder transition on grain boundary self-diffusion. For the experiments the equiatomic FeCo compound was chosen and the measurements were carried out in the B-kinetic regime of Harrison [2]. The diffusion was investigated by using radiotracers and the serial sectioning technique. The chosen temperature range, 873K-1133K, includes the critical temperature of the A2-B2 type order-disorder transition. The main results are the following: i) the triple-product values cannot be described by means of a simple Arrhenius type relationship; ii) there is a net breaking point on the Arrhenius plot, a somewhat below the A2-B2 transition temperature, which is in agreement with numerical simulations [3]; iii) the activation energies of diffusion are identical in the A2 and B2 regions for both solutes; iv) iron diffuses faster than cobalt in the disordered region, while in the ordered state this tendency is inversed. A natural outcome of these observations is an apparent change in the pre-exponential factor of the triple-product for both solutes. This behavior can only be interpreted by taking into account i) a change in the grain boundary structure implying a modification of the grain boundary width; ii) a self-segregation to grain boundaries of iron at high temperatures and of cobalt at low temperatures; iii) a change in the pre-exponential factor of the grain boundary diffusion coefficient due to correlation effects. At the moment there is no any reason to favor either of these possibilities. Further studies including chemical composition and structure determination on bicrystals would be of deciseve force.
1 H. Mehrer, Materials Transaction JIM, 37, 1259 (1996)
2 I. Kaur, Yu. Mishin and W. Gust, Fundamentals of Grain and Interphase Boundary Diffusion, Third enlarged edition, John Wiley and Sons Ltd. (1995)
3 M. El. Azzaoui, M. Hou and W. Pontikis, Interface Sc., 2, 57 (1994)

2:15 PM *Z5.3
GRAIN BOUNDARY DIFFUSION AND SEGREGATION IN SOLID STATE PHASE TRANSFORMATIONS. Eugen Rabkin, TECHNION, Dept of Materials Engineering, Haifa, ISRAEL; Wolfgang Gust, Institut für Metallkunde, University of Stuttgart, Stuttgart, GERMANY.

It is demonstrated that the discontinous ordering reaction in Fe-Co alloys provides the possibility to study the dependence of the grain boundary self-diffusion coefficient on the misorientational degrees of freedom. It is shown that vicinal grain boundaries with misorientations close to the special ones with a low value of the reciprocal density of the coincident sites exhibit also a low reaction rate and low self-diffusivity. The influence of segregating impurities on the parameters of grain boundary diffusion determined from the kinetics of the discontinuous precipitation reaction is considered in the framework of Cahn's solute drag theory. Cahn's model of the discontinuous precipitation reaction can be applied to the processing of the experimental data, however, the segregation factor has different values for stationary and moving grain boundanes, which should be taken into account dtmng the comparison of the grain boundary diffusivities obtained from the analysis of the discontinuous precipitation kinetics and from radiotracer measurements on stationary boundaries. The developed theory is applied to the analysis of the discontinuous precipitation reaction in the Ni-In and Cu-Bi-In systems, in which the segregation data for stationary boundaries are available.

2:45 PM Z5.4
MODELING THE EFFECT OF IRRADIATION AND POST-IRRADIATION ANNEALING ON GRAIN BOUNDARY COMPOSITION IN AUSTENITIC IRON-CHROMIUM-NICKEL ALLOYS. T. R. Allen, Argonne National Laboratory-West, Idaho Falls, ID; J.T. Busby and G.S. Was, Department of Nuclear Engineering and Radiological Sciences, University of Michigan, Ann Arbor, MI; E. A. Kenik, Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN.

Many irradiation effects in iron-chromium-nickel alloys such as radiation-induced segregation, radiation-enhanced diffusion, and void swelling are known to vary with bulk alloy composition. The development of microstructure and microchemistry in irradiated alloys is determined by the rate of diffusion of point defects and alloying elements. Additionally, the changes to microstructure and microchemistry that occur during annealing in these irradiated materials also depends on the rates of diffusion. To accurately predict the changes in grain boundary chemistry due to radiation-induced segregation and post-irradiation annealing, diffusion parameters such as the migration energy, must be known for each bulk composition. A model is developed which calculates migration energies using pair interaction energies, thereby accounting for the changes in diffusivities as a function of changing composition. The advantages of this calculation method are that a single set of input parameters can be used for a wide range of bulk alloy compositions, and the effects of local order can easily be incorporated into the calculations. A description of the model is presented, and model calculations are compared to segregation measurements from seven different iron-chromium-nickel irradiated with protons at temperatures between 200&#186;C and 600C and to doses from 0.1 to 3.0 dpa. Comparison of the model predictions is also made with segregation measurements from alloys irradiated with neutrons. Finally, model predictions are compared to grain boundary composition measurements of alloys annealed following irradiation. Comparisons indicate that the significant trends seen in the segregation and annealing can be modeled using a single set of input parameters with no error greater than 5 at%, but usually less than 2 at%. In addition to correctly predicting the changes in grain boundary chemistry in irradiated alloys, the model correctly reproduces self-diffusion coefficients in similar iron-chromium-nickel alloys.
This project supported by the Department of Energy under grant DE-FG02-89ER-7552 and by Pacific Northwest National Laboratory. Research was sponsored by U.S. Department of Energy, Division of Materials Sciences, under contract DE-AC05-96OR22464 with Lockheed Martin Energy Research Corp. and through the SHaRE User Program under contract DE-AC05-76OR00033 the Oak Ridge Institute for Science and Education.

3:30 PM Z5.5
ATOMISTIC SIMULATION OF VACANCY DIFFUSION WITHIN THE CORES OF A DISSOCIATED GLIDE DISLOCATION IN AN FCC METAL. R. G. Hoagland, Washington State Univ, Pullman, WA; S. M. Foiles, Sandia Nat. Lab., Livermore CA; and A. F. Voter, Los Alamos Nat Lab, Los Alamos, NM.

We have employed an elastic band method to explore the migration energies for various paths within the cores of two, closely-spaced, Shockley partials, one of which is pure edge, the other nearly screw. The calculations were performed using an EAM potential for aluminum with a stacking fault energy of about 125 mJ/m**2, and so the partials are separated by about 0.8 nm. Certain paths exhibit distinctly lower migration energies than others, as, for example, a zigzag path adjacent to the edge and a helical path around the mixed partial. Migration energies parallel to the dislocation line are significantly higher than in the perfect crystal. The migration energy for a vacancy along an infinite stacking fault is found to be slightly less than in perfect crystal. The migration energies for a large number of paths within the cores of these two partials were used as input to a kinetic Monte Carlo calculation to estimate vacancy diffusivity in the cores relative to the perfect crystal.
This work was supported by the United States Dept of Energy, Office of Basic Energy Sciences, Division of Materials Science under Grant DE-FG06-87ER45287.

3:45 PM Z5.6
DISLOCATION PINNING IN GAMMA-BASED TITANIUM ALUMINIDES DUE TO DIFFUSING DEFECT ATMOSPHERES. F. Appel, U. Christoph and R. Wagner, Institute for Materials Research, GKSS Research Center, Geesthacht, GERMANY.

Over the temperature range 150-350&#186;C thc deformation behaviour of gamma-based titanium aluminides is characterized by discontinuous yielding and a negative strain rate sensitivity. These phenomena are usually associated with the Portevin-LeChatelier effect, which arises from the dynamic interaction of diffusing defects with the dislocations. The mechanism provides a significant glide resistance and often leads to embrittlement of the material. The diffusion processes behind the mechanism were investigated on different single-phase and two-phase titanium aluminides by static strain aging. The experiments involve prestraining of samples followed by aging for certain periods of time. During the aging period a transfer of defects onto the mobile dislocations is expected so that locking occurs. On reloading distinct yield point phenomena were observed, which indicate that the dislocations were effectively pinned due to the formation of defect atmospheres. The observed fast kinetics and activation energies of the aging process will be discussed concerning the data on defect mobilities in gamma-TiAl known from the literature.

4:00 PM Z5.7
ON THE KINETIC MECHANISM OF GRAIN BOUNDARY WETTING IN METALS: SELF DIFFUSION INTO GB AS THE PERFECT SINK. Evgeny E. Glickman, Tel Aviv Univ., Faculty of Engineering, Tel Aviv, ISRAEL.

Based on a new insight into the GB wetting observations reported for solid-liquid metal systems a novel kinetic mechanism of complete wetting (CGBW) is proposed. It involves extension of GB groove drained by self- diffusion flux of ${\it ``solid``}$ atoms ${\it ``into the GB ``}$. The driving force for the flux stems from the imbalance in the GB and interphase surface tensions $\gamma_{gb}$ and $\gamma_{sl}$ acting at the groove root. The key physical assumption here is that instead of the equilibrium dihedral angle $\Theta_{0}$ - which under complete wetting conditions should be zero - a ``dynamical`` angle $\Theta_{d}$ is established and maintained at the root of advancing finger like groove to ensure here an intimate contact between the liquid and solid phases. This means that mechanical equilibrium at the root does not exist, that the linear force F$_{\gamma}$ = $\gamma_{gb}$ - 2$\gamma_{sl}$ cos($\Theta_{d}$/2) = $\gamma_{gb}$[1- cos($\Theta_{d}$/2)] points into the solid and its hydrostatic component can cause the diffusion flux into the GB considered to be a perfect sink and allowing grains to move apart normally to the GB plane. With this, the problem of the groove shape and kinetics is reduced essentially to that for GB grooving under an arbitrary GB flux (L. M. Klinger, E. E. Glickman, V. E. Fradkov, W. W. Mullins and C. L. Bauer, J. Appl. Phys. 78(6), 3833 (1995) ). It is shown that the model captures the major features of CGBW, namely, the concave groove profile, the groove width and velocity and the ``swelling`` effect. It explains also transition from partial to complete wetting which is expected to occur at T = T_w , when $\Theta_{0}$(T = T_w) =$\Theta_{d}$ and, therefore, F$_{\gamma}$ = 0. Finally, critical experiments for the verification of the novel GB self diffusion mechanism of CGBW are discussed.

4:15 PM Z5.8
EFFECT OF SURFACE SEGREGATION ON THE TEMPERATURE DEPENDENCE OF ION BOMBARDMENT INDUCED SURFACE MORPHOLOGY: AN STM INVESTIGATION OF A Cu-0.45 at.$\%$ Sb (111) ALLOY. Bernard Aufray, CRMC2-CNRS, Campus de Luminy, Marseille, FRANCE, Helene Giordano, Laboratoire SERMEC, Univ Aix-Marseille III, Marseille, FRANCE, Vania Petrova, Univ of Illinois, Center for Microanalysis of Materials, Urbana-Champaign, IL, David Seidman, Materials Science and Engineering Dept, Northwestern Univ, Evanston, IL.

It is well known that the ion bombardment of a metallic surface typically induces a surface morphology consisting of small clusters and craters of atomic dimensions that generally exhibit the symmetry of the surface crystallography. These surface defects can be eliminated very rapidly by a short anneal at an elevated temperature. Nevertheless, when the substrate is a metallic alloy with a solute that strongly segregates at the surface, two phenomena concomitantly occur during annealing: (a) surface diffusion which plays a role in eliminating the surface defects and obtaining large terraces, and (b) surface segregation of solute from the bulk which affects surface diffusion. We present a scanning tunneling microscopy (STM) study, performed at different elevated temperatures, of the influence of Sb surface segregation on the morphology of the (111) surface of a solid-solution Cu-0.45 at.$\%$ Sb single-crystal, cleaned at room temperature by Ar+ ion sputtering. Surprisingly, when the temperature is increased from room temperature (RT) to 400&#186;C, the typical (111) surface morphology, obtained after ion bombardment, evolves in very different way depending on the annealing time: if it is short (approximately a few minutes), it evolves to large terraces whereas if it is long (about 10 hours) it does not evolve (i.e., it is frozen). These surprising results are interpreted in terms of the point defects created during ion bombardment that are mobile and mediate Sb diffusion at low temperature, but could precipitate in the case of a rapid temperature rise, thereby forming small clusters near the surface in which Sb atoms are trapped.
Research at Northwestern was supported by the NSF under grant No. DMR-9419171.

4:30 PM Z5.9
A METHOD FOR EXAMINATION OF THE STRUCTURE, DYNAMIC, ELECTRONIC, MAGNETIC AND DIFFUSION PROPERTIES OF THE GRAIN BOUNDARY CORE AND ADJACENT REGIONS OF THE LATTICE. Semyon M. Klotsman, Vladimir N. Kaigorodov, Michail I. Kurkin, Institute of Metal Physics, Russian Academy of Sciences, Ekaterinburg, RUSSIA.

The proposed method [1] for examination of the structure, dynamic, electronic, magnetic and diffusion properties of the grain boundary core and adjacent regions of the lattice uses the nuclear gamma-resonance spectroscopy (NGRS) and intercrystalline diffusion (ICD) to populate by atomic probes the states, which are localized in the ICD zone. The temperature-time dependences of the states population in the ICD zone were shown to depend only on the pumping coefficient D_pump of the atomic probes from the grain boundary (GB) core and the segregation coefficient of the atomic probes at GB's. Only two discrete types of states are populated in the ⁵⁷Co ICD zone in all the polycrystalline cubiclattice metals, which have been studied up to date. The states-1 are localized in the GB core. The population of the states-1 only decreases with increasing annealing temperature. The states-2 are localized outside the GB core in the atomic probe pumping zone. The population of the states-2 increases only with the annealing temperature. From the dependence of the areas of the NGRS components on the measurement temperature it is possible to determine the dynamic properties (the Debye temperature) and detect a magnetic phase transformation in the vicinity of each of these states. The isomer shifts of the NGRS components and their dependence on the annealing temperature are used to ascertain types of the states and analyze the response of these states to alloying of the atomic probe localization regions in the ICD zone. In combination with the dependence of the NGRS component width on alloying of the atomic probe localization regions in the ICD zone, they permit determining the symmetry of point defects, which alloy the grain boundary core and adjacent two dimensional regions of the lattice.
[1]. Klotsman S.M., Sov. Phys. Uspech., 33(1), 55, (1990).

4:45 PM Z5.10
SELF AND IMPURITY DIFFUSION IN QUASICRYSTALS OF THE Al-Pd-Mn SYSTEM. Wolfgang Sprengel, Hideo Nakajma, The Institute of Scientific and Industrial Research, Osaka University, Osaka, JAPAN; Thomas Lograsso, Ames Laboratory, Iowa State University, Ames, IA.

In a systematic study we have investigated self- and impurity diffusion in icosahedral quasicrystals of the Al-Pd-Mn system (i-AlPdMn). The diffusivities were determined by the radioactive tracer method in combination with precision grinding as sectioning technique. The temperature dependence of ⁵⁴Mn, ⁶⁸Ge, ⁶⁰Co, and ³¹Cr diffusion was measured in the range from 853 to 1103 K. The diffusivities in i-AlPdMn cover several orders of magnitude and the temperature dependencies follow Arrhenius laws. Ge diffusion with an activation energy of 148 kJ/mol is about one order of magnitude faster than Mn self-diffusion. In contrast diffusivities of the transition metal elements Co and Cr with activation energies around 250 kJ/mol are two to three orders of magnitude smaller. From the results of the Mn self-diffusivities ¹⁾ a diffusion mechanism via vacancies seems very likely for i-AlPdMn. The data on impurity diffusion suggest different vacancy diffusion machanisms for Ge and the transition metal elements Co and Cr. Possible mechanisms involving different atomic sites in i-AlPdMn are presented and will be discussed.
1) W Sprengel, Th. A. Lograsso, and H. Nakajima, Phys. Rev. Lett. 77 (1996) 5233.

SESSION Z6: POSTER SESSION: DIFFUSION IN METALS, ALLOYS AND INTERMETALLICS
Chairs: Yuri Mishin and Gero Vogl
Tuesday Evening, April 14, 1998
8:00 P.M.
Salon 7

Z6.1
B & MD - THE COMBINED METHOD FOR SIMULATION OF DEFECT MIGRATION. Andrei Gusak, Tatyana Zaporoghets, Cherkasy State University, Dept. of Theoretical Physics, Cherkasy, UKRAINE.

Calculation of migration enthalpy for defect diffusion by MD simulation usually needs detecting many atom jumps and consequently large computation time. We propose to use Brownian dynamics for the defect in periodic potential with stochastic and viscous forces with characteristics derived from the direct MD simulation of defect motion. The coefficient of the viscous force and the amplitude of stochastic force are expresssed in terms of time correlation functions for periodic force calculated from the results of MD-simulation. Stochastic force is taken to be of gaussian type without any time correlation. This method saves computation time since the calculation of correlation functions needs less time intervals. Calculations have been made for interstitial light atoms in the copper lattice.

Z6.2
MD-SIMULATION OF DEFECT MIGRATION AND GENERATION IN THE SHOCK WAVE. Andrei Gusak, Tatyana Zaporoghets, Cherkasy State University, Dept. of Theoretical Physics, Cherkasy, UKRAINE.

Numerous experiments on masstransfer and phase transformations in alloys under the shock waves show the anomalously large penetration depth of atoms and sometimes anomalously fast formation of intermediate phases. These results can not be described in the terms of high diffusivities as well as mechanical diffusion. Evidently, this effect is linked with defect generation and migration in solids under the shock wave. At the present time microscopic theory of these processes is absent. Therefore it is important to investigate the behavior of defects under the shock wave. We simulated a shock wave in copper with different types of defects using GibbsonII potential of Born-Mayer type. Four neighbor shells for defects were taken into account. Shock wave was initiated by uniform motion of two boundary atom layers. Interstitial atoms appeared to be moved by the shock wave for sufficiently large SW velocity. Moreover, the generation of new defects (interstitials, vacancies, dumb-bells) had been detected just behind the front of the shock wave in the vicinity of interstitial. Kick-out mechanism was realized. Local reconstruction of lattice in the vicinity of interstitial, initiated by the SW, was observed.

Z6.3
THE KIRKENDALL EFFECT AND EXTERNAL STRESS GRADIENT: MONTE CARLO MODELING. Michael V. Yarmolenko, Institute of Fire Safety, Dept. of Fire Protection, Cherkasy, UKRAINE.

A computer code was worked out to model the Kirkendall effect during mutual diffusion between two solid substances under external stress gradient by Monte Carlo calculations. It takes into account the random vacancy jumps in a cubic lattice, appearance and disappearance of vacancies due to climbing of dislocations to be parallel to the initial interface, vacancy quasi-equilibrium in all points of each substance, different frequency of exchange of vacancy and the atoms of different kinds, and external stress gradient. We solved a random-walk vacancy problem with appropriate boundary conditions. The main assumption is P_B/P_A=const>1. Here P_A (P_B) is the probability that an A-atom (a B-atom) will jump into any given neighboring vacant site. External stress gradient influence is introduced as different probabilities of vacancy jumps in substance A and substance B directions. Monte Carlo modeling shows that there are deviations from the parabolic law of the shift of inert markers, and the time rate of change of the shift decreases exponentially with distance increasing from the initial interface. The results of computer modeling are compared with real experimental data.

Z6.4
SIMULATIONS OF DEFECT AND DIFFUSION PROPERTIES IN THE INTERSTITIAL Cu-C SOLID SOLUTIONS. Donald Ellis, Kleber Mundim, Department of Physics and Astronomy, Northwestern University, Evanston, IL; Simon Dorfman, Dept of Physics, Israel Institute of Technology-Technion, Haifa, ISRAEL; and David Fuks, Mater. Eng. Dept., Ben-Gurion University of the Negev, Beer Sheva, ISRAEL.

The atomistic simulations in the framework of the Generalized Simulated Annealing approach (GSA) and classical force fields lead to very reasonable relaxed geometries around the carbon interstitial in O-, T-, and TS-sites. We have thus shown that a highly efficient energy-sampling and relaxation scheme, implemented with tight constraints on a limited volume, provides a powerful steering mechanism for selection of geometries suitable for detailed investigation by first-principles methods. The present results, based upon harmonic interactions between Cu atoms and a van der Waals interaction between Cu and C, predict the relaxed O-site to be more stable than the T-site by 1.2 eV, in accordance with general expectations. The TS barrier to O O diffusion is found to be 0.8 eV, at a temperature of 0 K; the TS exhibits a strong local axial distortion of the pseudo-octahedral environment. The Density Functional results indicate a charge transfer of 1 e to carbon, mostly from the first neighbor shell, in all relaxed environments studied. Bond-order data show the Cu-C interaction to be bonding in nature, despite the net `repulsive interaction' leading to a surface state of lower net energy.

Z6.5
INTERCONNECTION BETWEEN SELF- AND REACTION-DIFFUSION COEFFICIENTS IN BINARY SYSTEMS. Vasil I.Dybkov, Inst for Problems of Materials Science, Dept of Physical Chemistry of Inorganic Materials, Kyiv, UKRAINE.

Solid state diffusion is an important factor in many technological processes (welding, brazing, soldering, all-in-one joining of dissimilar metals, making very-large-scale integrated circuits, synthesis of inorganic compounds, etc.). To evaluate their kinetics, it is necessary to know the values of diffusion coefficients of the elements involved into the interaction. If the reaction product is a chemical compound (an intermetallic, a silicide, an oxide, a salt, etc.) that forms a coherent layer at the interface between initial substances, then its formation is associated with the reaction-diffusion coefficients of the components in that compound. However, what is more often available in the literature is the data on self-diffusion coefficients of the constituents of chemical compounds. The values of the self- and reaction-diffusion coefficient of the same element in the same compound are known to be different, the first being far less (by a few orders of magnitude) than the second. When using the value of the self-diffusion coefficient for evaluating the compound layer-growth rate, it is necessary first to normalise this value to the vacancy concentration at which it was measured. Analytical expressions which can be used for this purpose are analysed. The results of calculations for different compounds (intermetallics, silicides, oxides, etc.) are presented to show that after the normalisation the data on self-diffusion coefficients in binary systems can readily be used to evaluate at least the order of magnitude of the reaction diffusion kinetics.

Z6.6
INFLUENCE OF THE CONSTANT MAGNETIC FIELD ON IMPURITY DIFFUSION OF NICKEL-63 IN POLYCRYSTALLINE COBALT. Dinis Mironov, Alexander Pokoev, Samara State Univ, Dept of Physics, Samara, RUSSIA.

Recently in works [1,2] data on appreciable influence of the constant magnetic field (CMF) on impurity diffusion of nickel-63 in polycrystalline ýrmco- and monocrystalline siliceous iron has been received. In the present work with the purpose of finding-out of mechanisms of CMF influence on impurity diffusion in ferromagnets and accumulation of the experimental facts in this direction research of nickel-63 impurity volume diffusion in polycrystalline cobalt has been realized. Measurements have been carried out by the residual activity method in intervals of temperatures 1163-1543 K, diffusion times 0.18-42 hours and tensions of CMF 0-400 Í¿/Ï. The grain size in samples polycrystalline cobalt made 2-3.5 mm; an experimental error of measurements of diffusion coefficients (DC) is 3-10 %. The analysis of received results shows, that in absence CMF in the region of the cobalt point Curie appreciable diffusion magnetic anomaly of nickel-63 in cobalt is observed. The size of effect is different from t

Z6.7
MECHANISMS OF THE IMPURITY DIFFUSION IN FERROMAGNETIC METALS IN THE CONSTANT MAGNETIC FIELD. Alexander Pokoev, Dmitrii Stepanov, Igor Trofimov, Samara State University, Dept of Physics, Samara, RUSSIA.

Earlier by means of the residual activity method it was established that the constant magnetic field (‹MF) influences noticeably the nickel and the aluminium volume diffusion in the polycrystalline cobalt, armco- ýnd monocrystalline siliceous iron at the ferromagnetic temperatures. In this paper some possible mechanisms of the impurity diffusion in ferromagnetic metals in the CMF has been analysed: a) diffusion of the impurity atoms, which have magnetic moment, under the tension CMF gradient; b) diffusion in matrix with different degree of the magnetic ordering ; c) diffusion in ferromagnetic matrix, which is being deformed by the CMF (magnetostriction mechanism) and d) diffusion in matrix with changed crystal structure defects density in the magnetic field. The CMF influence on the impurity diffusion in the ferromagnetic metals was estimated for all named mechanisms. The expression for the diffusion coefficient was received for the diffusion of the impurity atoms, which have magnetic m

Z6.8
THE VOLUME DIFFUSION OF INTERSTITIAL ATOMS IN ORDERED MAGNETIC ALLOYS UNDER PRESSURE Svetlana Zaginaichenko, Inst for Materials Science Problems of UAS, Kiev, UKRAINE; Zinaida Matysina, State Univ, Dnepropetrovsk, UKRAINE.

Theoretical investigation of phenomenon of the interstitial atoms diffusion in alloys with atomic and magnetic orders under all-round external pressure has been performed. It has been studied the simultaneous influence of substitutional atoms ordering and spontaneous magnetization with ferro- and antiferromagnetic orders with due regard for high external pressure on the interstitial atoms diffusion. The atomic ordering has been examined in Bragg-Williams-Gorski approximation, the magnetism - in q uasi-classical Heisenberg approximation. It has been taken into account two mechanisms of influence of alloy magnetization on diffusion of interstitial atoms: 1) mechanism of influence through atomic ordering and 2) strictional mechanism. Formula for the diffusion coefficient determine its dependence on alloy composition, temperature, pressure, order parameters and energetic constants. It has been investigated the particular cases: 1) nonmagnetic alloy with atomic ordering when no external pressure exists and 2) ferromagnetic alloy with atomic ordering. The plots of temperature and concentration dependences of diffusion coefficient D(T), D(c) have been constructed. It has been shown that in Kurnakov and Curie points the diffusion coefficient has a discontinuity or break at the phase transition of the first or second kind, respectively. The atomic order decrease the coefficient of diffusion. The concentration dependence of diffusion coefficient for ordered ferromagnetic alloys is extremal. The experimental construction of D(T) and D(c) can permit to evaluate the Kurnakov, Curie and Neel temperatures and to estimate the kind of phase transition.

Z6.9
SURFACE SELF-DIFFUSION IN ORDERED MAGNETIC ALLOYS. Svetlana Zaginaichenko, Inst for Materials Science Problems of UAS, Kiev, UKRAINE; Zinaida Matysina, State Univ, Dnepropetrovsk, UKRAINE; Dmitriy Schur, Inst of Hydrogen and Solar Energy, Kiev, UKRAINE.

Theoretical investigation of phenomenon of Gibbs enrichment of surface layer of crystal face (110) by one of alloy components, its influence on the equilibrium numbers of vacancies, on self-diffusion coefficient in binary ordered magnetic alloy AB with body-centered cubic structure have been performed. The calculation of free energy has been carried out by the method of average energies in Gorski-Bragg-Williams model of atomic ordering and in quasi-classical Heisenberg model of magnetism in approximation of pair electrochemical and exchange interaction of nearest atoms. It has been taken into account the possibility of phase transitions of atomic order-disorder and paramagnetic-ferromagnetic (antiferromagnetic) material types. As it has turned out the surface numbers of vacancies differ from the such for volume both in concentration dependence and in dependence on parameters of atomic and magnetic order. The coefficient of surface self-diffusion of labelled atoms A* has been calculated for disordered ferromagnetic alloys. Its has been found to be essentially different from the such in the absence of surface segregation and the such for volume. This difference shows up in dependence on alloy composition and magnetization parameter. The analytical calculations have been illustrated by numerical examples. It have been constructed the phase diagram of magnetic alloy, the plots of surface concentration of A component in dependence on initial concentration of this component, temperature, parameter of magnetic order and also the plot of temperature dependence of self-diffusion coefficient. The curve break become apparent on the last in the surface Curie point (which is higher than volume point). The received formulae can permit to predetermine for each alloy the possibility of segregation manifestation, to predestine the temperature dependence of surface self-diffusion coefficient if the energetic constants are known from independent experiments.

Z6.10
Abstract withdrawn.

Z6.11
CROSS-STRIP ELECTROMIGRATION STUDIES OF CU-AL, CU-SN FILMS. N.L. McClanahan, C.-U. Kim, S. Naik, H.D. Yang, Materials Science and Engineering Department, University of Texas at Arlington, Arlington, TX.

With increasing demand for reliable and high performance devices, interest in interconnection technology based on Cu thin films is increasingly keen. Although it has been known that Cu is highly resistant to electromigration damage, the mechanism of electromigration in Cu is less well understood. In particular, quantitative analysis of solute electromigration in Cu is a matter of research interest for several reasons. In order to better understand the mechanism for electromigration in Cu, we focused on the electromigration nature of two particular solutes, Al and Sn. Al and Sn were each deposited over the central portion of a Cu line in a cross-strip configuration and then subjected to electromigration conditions. After testing at several temperatures, samples were analyzed to determine the effective valence and diffusivity of Al and Sn. Although the specific mechanism involved is still unknown, findings suggest that Sn migrated toward the cathode while Al migrated toward the anode. This interesting result indicates that continued study of Cu alloys may lead to a better understanding of the role of solutes in electromigration, which has been a subject of research for several decades.

Z6.12
TDPAC STUDY OF Zr-Hf ALLOYS. F. Dyment, E.D. Cabanillas, Materials Department, Atomic Energy Commission, Buenos Aires, ARGENTINA and L. Mendoza Zelis, L.C Damonte and A.F. Pasquevich, Physics Department, La Plata University, La Plata, ARGENTINA.

In this work Time-Differential Perturbed Angular Correlation (TDPAC) studies of the alpha to beta transition in dilute Zr-Hf alloys (1.3 and 4.5 at%Hf) are presented. The electric field gradient, measured through the perturbation experienced by 181Ta atom probes, is related to the local modifications of the charge distributions around Hf sites. In this way the atomic changes during the hexagonal to cubic transition are investigated with the aim to relate them to the diffusion anomalies observed in the alpha Zr phase. The obtained temperature dependence of the electric field gradient shows a departure of the usual T3/2 law when the transition temperature is reached from below. This results will be discussed in connection with the mentioned diffusion anomalies.

Z6.13
POSITRON STUDIES OF DEFECTS IN NITROGEN IMPLANTED TITANIUM. Maziar Soltani-Farshi, Horst Baumann, Klaus Bethge, Institute for Nuclear Physics, Frankfurt, GERMANY; Wolfgang Anwand, G. Brauer, Research Center Rossendorf, Dresden, GERMANY; Dorothee M. Rueck, Center for Heavy Ion Research, Darmstadt, GERMANY; P. G. Coleman, School of Phsysics, University of East Anglia, Norwich, UNITED KINGDOM.

Modification of materials by ion beams creates defects and defect formation processes. The implantation of nitrogen into titanium influences the hydrogen content in this metal. Hydrogen accumulation enhances hydrogenations in the implanted region. This effect may have important consequences, because excessive hydrogen accumulation generally leads to precipitating hydrids in the matrix lattice and the metal undergoes degradation of its mechanical properties. Many studies have shown that defects in metals trap light gas atoms like H or He which are solved or implanted in the sample. Therefore, the decoration of defects with these atoms is a method to trace defect concentrations and to study the trapping and detrapping mechanism. Mobile defects can be trapped at implanted atoms, at inhomogeneities or at inner surfaces like grain boundaries or interfaces of different phases. Using the slow positron beam technique the traps for hydrogen attributed to vacancy-type defects have been investigated in cp-Ti implanted with different fluences of N and at varying temperatures. First results are presented. The concentration of hydrogen has been detected by the ¹⁵N profiling technique (¹H(¹⁵N,$\alpha \gamma$)¹²C) and the nitrogen content by the inverse reaction.

Z6.14
DIFFUSION OF CARBON INTO RE₂Fe₁₇ LATTICE: A HIGH TEMPERATURE APPROACH. M. Venkatesan^a, U.V. Varadaraju^a and K.V.S. Rama Rao^b; ^aMaterials Science Research Centre, ^bMagnetism and Magnetic Materials Laboratory, Department of Physics, Indian Institute of Technology, Madras, INDIA.

Nitrogen or carbon insertion into rare-earth intermetallics, such as R₂Fe₁₇ by gas-phase reaction technique has recently attracted considerable attention due to the application of the resulting nitrides/carbides as permanent magnet materials[1]. However, the abovesaid interstitial compounds are thermodynamically unstable and decompose into rare earth nitrides (or carbides) and $\alpha$-Fe at high temperatures. Resently, it has been reported that highly stable R₂Fe₁₇C_x compounds can be formed by the substitution of Ga for Fe. In addition, the substitution of Ga facilitates the formation of interstitial carbides with high temperature stability directly by arc melting contrary to the metastability of the gas-phase interstitially modified compounds. The compounds with composition HoErFe₁₅Ga₂C_x (x - 0.5, 1.0, 1.5, 2.0) werc synthesized by arc melting the high purity starting elements followed by annealing at 900&#186;C for 7 days. Powder X-ray diffraction measurements were performed on the compounds using CrK$\alpha$ radiation ($\lambda$ - 2.289 Å) to identify the phase purity and the crystallographic structure. The prepared samples are single phase with the hexagonal Th₂Ni₁₇ type structure. The unit cell volume increases linearly with carbon content and the maximum volume expansion is found to be 6.2% for x=2.0 when compared to HoErFe₁₇. The Curie temperature increases from 480 K for x=0 to 587 K for x=2.0, which can be explained using magnetic exchange coupling mechanism. X-ray diffraction measurements on magnetically aligned powder samples show that there is a change in easy magnetization direction (EMD) from basal plane to cone which can be explained in terms of change in the magnitude of second order crystal field parameter A₂₀, a dominant factor in determining the rare earth sublattice anisotropy.
References
1. J.M.D. Cocy and H. Sun J. Magn. Magn. Mater. 87 (1990) L251.

Z6.15
MECHANISMS OF THERMAL ACTIVATED MOBILITY OF INTERSTITIAL CLUSTERS IN FE AND CU. Yuri Osetsky, Liverpool Univ, Dept. of Mat. Sc. and Eng., UNITED KINGDOM; Anna Serra and Vicenc Priego, Univ. Politecnica de Catalunya, Dept. Matematica Aplicada III, Barcelona, SPAIN.

An extensive molecular dynamics simulation of clusters mobility from 2 to 24 interstitials has been carried out in Cu and Fe in a temperature range from 400 to 1200K. The results obtained for Cu shown that clusters bigger than 2 interstitials are one-dimensionally mobile in the <110> direction. In Fe all clusters are one-dimensionally mobile in either <111> or <100> directions. It was found that in both metals the migration energy of clusters does not depend on the number of interstitials although the jump frequency decreases for big clusters. A new mechanism of interstitial clusters migration is suggested. A simple expression for the estimation of the cluster jump frequency has been obtained. The diffusion coefficient has been estimated within the one-dimensional
random walk model.

Z6.16
SOLID STATE DIFFUSION AND REACTIONS DURING INTERMETALLIC FORMATION IN THE Ni-Bi AND Ni-Zn BINARY SYSTEMS. Oleg V.Duchenko, Vasil I. Dybkov, Inst for Problems of Materials Science, Dept of Physical Chemistry of Inorganic Materials, Kyiv, UKRAINE.

Comparative investigation of the diffusion mechanisms and intermetallic layer-growth kinetics has been performed in the Ni-Bi and Ni-Zn binary systems. For the Ni-Bi couple, only the NiBi₃ layer was found to grow at 150 to 250&#186;C while two intermetallics NiBi and NiBi₃ are known to exist in the Ni-Bi binary system according to its phase diagram. In contrast, two compound layers NiZn₃ and Ni₃Zn₂₂ were observed to form at 250 to 400&#186;C in the Ni-Zn reaction couple, the total number of intermetallics on the phase diagram being four. The growth kinetics in both Ni-Bi and Ni-Zn systems have been studied with the use of microhardness indentation markers to reveal the main diffusing species. The layer compositions were determined by means of electron probe microanalysis. While the intermetallics NiBi₃ and Ni₃Zn₂₂ are stoichiometric, the NiZn₃ phase has a relatively wide range of homogeneity. The growth process has been found to be diffusion controlled for all the intermetallic layers. The NiBi₃ layer increases entirely at the Ni-NiBi₃ interface, with bismuth being the dominant diffusant. Values of the bismuth diffusion coefficient were determined at 150 to 250&#186;C. Its temperature dependence obeys the Arrhenius equation. The diffusion in the NiZn₃ and Ni₃Zn₂₂ phases are more complicated. Values of the diffusion coefficients of the constituent elements in growing layers were determined experimentally and discussed from the point of view of existing diffusion mechanisms. It is noteworthy that mechanical stresses due to the volume change associated with the large difference in the molar volumes of the intermetallics of the Ni-Zn binary system strongly affected the layer-growth kinetics. If the specimen geometry was such that the stress arisen acted non-symmetrically, the disruption of the specimens along the Zn-Ni₃Zn₂₂ interface was observed. Moreover, the Ni phase was pushed out of the Zn matrix that was visible after prolonged annealing with the naked eye.

Z6.17
ANALYSIS OF IMPURITY DIFFUSION IN ORDERED ALLOYS AND INTERMETALLIC COMPOUNDS OF THE L1₂ TYPE. Irina V. Belova and Graeme E. Murch, Department of Mechanical Engineering, The University of Newcastle, Callaghan, NSW, AUSTRALIA.

Recently, there has been considerable interest in the measurement of impurity diffusion coefficients in binary alloys and intermetallic compounds. These initiatives have not however been underpinned by theoretical analysis along the lines that have been extensively developed for impurity diffusion in pure metals. The present authors have recently developed a general kinetics formalism to describe self and chemical diffusion in ordered alloys and intermetallic compounds covering the B1/B2, L1₂, A15 and DO₃ structure types. An extension to cover impurity diffusion in the B2 structure has recently been made. Those ideas are extended in this paper to the L1₂ structure where the added complication of parallel inter and intrasublattice jumps is dealt with. To illustrate the new formalism we make application to the extensive impurity diffusion measurements in Ni₃Al and Ni₃Ge.

Z6.18
MODELING OF DIFFUSION IN ORDERED STRUCTURES OF B2-TYPE. Maria G. Ganchenkova, Andrei V. Nazarov, Dept. of Materials Science, Moscow State Engineering Physics Institute, Moscow, RUSSIA.

Using Monte Carlo simulation, we model the interaction effect of vacancies to their diffusivities in ordered structures of B2-type. The activation barriers are calculated by a variation method using interatomic potentials for both vacancy exchanges: with nearest-neighbor atoms and next-nearest-neighbor atoms. It was made for any position of the second vacancy. Knowing these barriers it is possible to calculate the jump rates and to model the vacancy migration. The calculations were attributable to the system with parameters similar to NiAl. Conclusions: 1) In case of the ordered B2-type structure the vacancies are tending to form a dumb-bell shaped figure, which is stable within certain intervals of the parameters change. 2) Interaction between the vacancies positioned in different sublattices results in diffusion acceleration. 3) The modeling allows to assume the existence in ordered structures of B2-type of a new diffusion mechanism, which could be attributable to dynamic pair of vacancies and vacancy exchanges with next-nearest-neighbor atoms.

Z6.19
STOCHASTIC VACANCY MOTION IN NICKEL AND IRON ALUMINIDES DETECTED BY PAC. Mingzhong Wei, Bin Bai and Gary S. Collins, Dept of Physics, Washington State Univ, Pullman, WA; and William E. Evenson, Dept of Physics, Brigham Young Univ, Provo, UT.

We have been using the method of perturbed angular correlation of gamma rays (PAC) to detect and identify signals caused by point defects near probe atoms in NiAl, FeAl and other compounds having the B2 (CsCl) structure.[1] Well-resolved signals are observed for In/Cd probes on the Al-sublattice having one transition-metal vacancy in the first-neighbor shells in these systems. Here, we report recent measurements on NiAl and FeAl at high-temperature in which nuclear relaxation of both the vacancy signal and the signal from probes without nearby vacancies was detected. The relaxation appears as ì dampingî of the time-domain PAC signal, and is attributed to vacancies jumping near the probe atoms. The onset of relaxation occurs at a much lower temperature in FeAl (1100 K) than in NiAl (1500 K). The relaxation studied to date is in the slow-fluctuation regime, in which defect-induced electric-field gradients change stochastically either in orientation (e.g., when a trapped vacancy jumps to an equivalent site in the first-neighbor shell) or in magnitude (e.g., when a vacancy jumps into or out of the first shell). Thus, observed relaxation of the vacancy signal could in principle be due either to trapping, detrapping or reorientation, whereas the vacancy-free signal can be relaxed only by trapping of vacancies. Preliminary analysis suggests that relaxation mechanisms in NiAl and FeAl are the same: in each system the vacancy signal is relaxed by detrapping of vacancies and not by reorientation, and the vacancy-free signal is relaxed by trapping. Measurements underway to determine activation enthalpies for the jump processes will be reported at the meeting. This work was supported in part by the National Science Foundation under grant DMR 96-12306.
[1] G.S. Collins, P. Sinha and M. Wei, Hyperfine Interactions C(1), 380 (1996).

Z6.20
INVESTIGATION OF THE DIFFUSION JUMP PROCESS IN FEAL BY MONTE CARLO SIMULATIONS. Richard Weinkamer, Peter Fratzl, Bogdan Sepiol, Gero Vogl, Institut für Materialphysik, Universität Wien, AUSTRIA.

Atomistic Monte Carlo simulations on a rigid lattice have been performed in order to investigate the diffusion process in a binary alloy with B2 order. We considered an ABV-pair interaction model, where A, B and V denotes an A-atom, a B-atom and a vacancy, respectively. The chosen ordering energies are taken from neutron scattering experiments and ensure a phase diagram close to that of the Fe-Al system. The dynamics proceeds exclusively via exchanges of the vacancy with nearest neighbor atoms. Our simulations are motivated by measurements on Fe₅₀Al₅₀ done by quasielastic M{ossbauer spectroscopy [1]. From the obtained line broadening we earlier deduced an unexpected diffusion jump process of the iron atoms. In the simulations we determined the autocorrelation function of the iron atoms and its Fourier transforms allowing a direct comparision with the experimental data. The variation of free parameters, such as the temperature, the asymmetry parameter, etc., lead to a reasonable agreement with the experimental results. [1] G. Vogl and B. Sepiol, Acta Metall. Mater. 42, 3175 (1994).

Z6.21
INITIAL DISORDERING RATES IN IRRADIATE CU3AU AT ELEVATED TEMPERATURE: SIMULATION AND EXPERIMENTS. L.C. Wei, M. Ghaly, P. Bellon, C.P. Flynn, R.S. Averback, Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, IL.

Ordering-disordering of intermetallic compounds under ion irradiation is a basic problem of radiation effects, and the understanding of this effect could be very helpful to the ion materials processing. The ordering-disordering of Cu3Au has be an old subject, but recently it was shown that the initial disordering rate of Cu3Au under ion irradiation, surprisingly, is strongly temperature dependent. The initial disordering rate increases as the sample temperature increases, and reaches the maximum just below the critical temperature. There could be several possible explications of such experimental results. We have performed Molecular Dynamics and Monte Carlo simulations to get more understanding of this phenomenon. These simulations show that the combination of the thermal spike of the initial cascade and the out-diffusion of the vacancies from inside the cascade can explain the experimental results.

Z6.22
APPLICATION OF THE NEW MICROSCOPIC KINETIC APPROACH TO CALCULATION OF THE PHENOMENOLOGICAL COEFFICIENTS IN ORDERED ALLOYS. Vladimir Yu. Dobretsov, Russian Research Center Kurchatov Institute, Moscow, RUSSIA; Maylise Nastar, Georges Martin, DECM/SRMP, CE Saclay, FRANCE.

The new microscopic kinetic approach early suggested for homogeneous alloys is generalized to calculation of the phenomenological coefficients for matter transport in ordered alloys. This approach is based on the master equation which describes time evolution of the distribution function of a system. At equilibrium the distribution function is defined by the configurational Hamiltonian depending on physical interactions between atoms. To describe non-equilibrium states we used the same distribution function but with an effective Hamiltonian including time-dependent effective interactions consistently determined from kinetic equations. Using the same approximation as for homogeneous alloys, namely taking into account only pairwise effective interactions, we have written and solved the kinetic equations for a steady state closed to equilibrium. We considered binary ordered alloys with two sublattices where the matter transport is controlled by the vacancy-mediated diffusion mechanism. For such systems we obtained the microscopic analytical expressions of the diagonal and cross phenomenological coefficients. These expressions depend only on the physical interactions, the alloy composition on sublattices and the geometric correlation factor which reflects the crystallographic structure. The validity of these approximated expressions have been checked with Monte Carlo simulations based on the Allnatt's time-correlation method. Comparison with results obtained from both Monte Carlo simulations and phenomenological models are discussed.

Z6.23
ON MECHANISMS OF DIFFUSION ALONG GRAIN BOUNDARIES NEAR $\Sigma$5 MISORIENTATION IN COPPER. Sergei Prokofjev, Institute of Solid State Physics, Russian Academy of Sciences, Chernogolovka, Moscow, RUSSIA.

In our previous works the misorientation and temperature dependences of the diffusion of nickel [1, 2] and gold [3] along <100> symmetric tilt boundaries in copper near $\Sigma$5 misorientation were studied. The data were analysed in terms of the grain boundary (GB) structure. It was shown that the misorientation behaviour of the GB diffusion is in good agreement with predictions of the secondary GB dislocations model [4, 5]. Therefore, this model was used to determine the diffusion parameters of the $\Sigma$5$\{310\}$ boundary and of the cores of the SGBDs. Besides, the indications of the abrupt change of the behaviuor of the Arrhenius dependences were found. In the present study the data obtained in [1-3] are tested for simple (bulk-like) monovacancy mechanism. The analysis of the diffusion along $\Sigma$5$\{310\}$ boundary in the low temperature region suggests the possibility of the diffusion mechanism other than the simple monovacancy one. The diffusions along $\Sigma$5$\{310\}$ boundary in the high temperature region, along the cores of the SGBDs and along general type GBs reasonably satisfy the criterion of the simple monovacancy mechanism good enough. However, it is not enough to validate the bulk-like mechanism. Indeed, a number of the facts suggest that bulk and GB diffusion mechanisms differ essentially [6, 7].
This study is supported by the Russian Foundation for Basic Researches (96-02-18840H).
1. A.N. Aleshin, S.I. Prokofjev, Poverchnost', No. 9, 131, (1986). In Russian.
2. A.N. Aleshin, S.I. Prokofjev, L.S. Shvindlerman, Def. Diff. Forum 66-69, 861 (1989).
3. E. Budke, Chr. Herzig, S.Prokofjev, L.Shvindlerman, Def. Diff. Forum (1998). To be published.
4. D.G. Brandon, Acta Metall. 14, 1479 (1966).
5. W. Bollmann, Crystal Defects and Crystalline Interfaces, Springer, Berlin (1970).
6. B.S. Bokstein, Ch. V. Kopezky, L.S. Shvindlerman, Thermodynamics and Kinetics of Grain Boundaries in Metals, ''Metallurgija'', Moscow (1986). In Russian.
7. B.S. Bokstein, Def. Diff. Forum 66-69, 631 (1989).

Z6.24
DEFECT FORMATION AT INTERNAL INTERFACES AND GRAIN BOUNDARIES OF COLD-ROLLED Fe AND Ni DURING HEAVY ION IRRADIATION. G. Aggarwal and P.Sen, Jawaharlal Nehru University, New Delhi, INDIA.

Heavy ion irradiation at high energies is fast developing as a tool for materials modification. To understand mechanisms leading to defect formation and their diffusion in cold-rolled Ni and Fe, we have performed on-line 4-probe resistivity measurements of thin foils during 100 MeV oxygen ion irradiation, at 300K. The overall increase and saturation of resistivity with ion fluence is marked by discontinuous jumps. These jumps are similar to previous reports assigning them to atomic rearrangements[1]. Employing measurements at 80K and also studying the ion flux, sampling current and dependence on specimen microstructure, we assign the resistivity increase to atomic rearrangements at the internal interfaces or grain boundaries of these materials and not to usual point defect formation [2]. Using x-ray topography (XRT) as a tool to map strain at an artificial interface, and with supporting evidence from x-ray diffraction (XRD), we propose a model for the processes leading to the rearrangements. The entire sequence in the 4-probe measurement in presence of the ion beam, and on their removal as well, is shown as dissipation of the incident ion energy into new atomic configurations followed by a positive feedback and dissipative structure formation.
References
1. T.N. Todorov & A.P. Sutton, Phys. Rev. Lett. 70, 2138 (1993)
2. P. Sen, G. Aggarwal & U. Tiwari, accepted in Phys. Rev. Letters

Z6.25
Abstract Withdrawn.

Z6.26
PROPERTIES OF IRON ATOMS AT GRAIN BOUNDARIES IN Fe AND Fe₃Al. Oldrich Schneeweiss, Ilja Turek, Jirí Cermák, Institute of Physics of Materials, AS CR, Zizkova, Brno, CZECH REPUBLIC; Pavel Lejcek, Institute of Physics, AS CR, Praha, CZECH REPUBLIC.

Grain boundaries in Fe bicrystal and Fe₃Al polycrystals have been investigated using Emission Mössbauer Spectroscopy. Grain boundaries were decorated by atoms of a mother Mössbauer isotope ⁵7Co atoms (decaying into ⁵⁷Fe) using low temperature diffusion at which intergranular penetration was dominant. To avoid the effect of atoms on free surfaces of bicrystals and polycrystals, single crystalline samples of Fe and Fe₃Al with the ⁵⁷Co atoms on the surface were also studied. The hyperfine parameters were interpreted using comparison of experimental data with results of theoretical calculations of electron structure. The results show, that approximately 5% of the atoms at the {112} coherent twin boundary in pure iron exhibit different atomic surrounding than atoms in the bulk. On the other hand, about 74% of diffusing atoms in Fe₃Al occupy positions different from the equilibrium positions in the bulk.

Z6.27
RESPECTIVE ROLES OF SURFACE, GRAIN BOUNDARY AND VOLUME DIFFUSIONS IN DRIVING STRUCTURAL, MICROSTRUCTURAL AND MAGNETIC PROPERTIES OF MBE ALLOY THIN FILMS. Veronique Pierron-Bohnes, Mireille Maret, Laziz Bouzidi and Marie-Claire Cadeville, IPCMS-GEMM, Strasbourg, FRANCE.

The co-deposition of two metals using a molecular beam epitaxy (MBE) technique at various growth temperatures (T_G) yields single-crystal alloy thin films with a columnar microstructure whose structural and magnetic properties can be different from those of the corresponding equilibrium bulk alloys. A general overview of results obtained in Co-Ru and Co-Pt thin films will be presented. As an example, the hcp Co₃Ru and Co₃Pt films grown on Ru(0001) display a long range chemical ordering (LRO) along the growth direction. The periodicity of this LRO is twice that of the hcp lattice and its intensity is strongly dependent of T_G passing through a maximum at respectively 600 and 650 K. This LRO that does not exist in equilibrium phases is explained as resulting from the competition between two phenomena occurring simultaneously during the growth process : a surface effect driven by surface interactions and surface diffusion that tends to enrich the surface layer in the heavier element (Ru or Pt segregation) and a bulk effect driven by bulk interactions and bulk diffusion that tends to restore the bulk equilibrium phase when the bulk diffusion becomes efficient during the growth time. A thermally activated model that takes into account both effects is proposed. It reproduces quite satisfactorily the T_G dependencies of the LRO in Co₃Ru and Co₃Pt and that of the uniaxial magnetic anisotropy in CoPt₃, yielding activation energies for the diffusion that are clearly lower at surface than in bulk, as expected. At high T_G (above 800 K) or after ex situ anneals, the diffusion of the Ru buffer through the grain boundaries of the columnar microstructure, that occurs before the inside grain diffusion, isolates the columns magnetically and is responsible of a large coercive field.

Z6.28
DIFFUSION ASSISTED DISLOCATION CLIMB IN INTERMETALLIC GAMMA-TiA1. F. Appel and R. Wagner, Institute for Materials Research, GKSS Research Center, Geesthacht, GERMANY.

Titanium aluminide alloys based on the intermetallic gamma-TiAl phase have received increasing attention over the past decade due to their potential as high temperature structural materials. However, for technical applications the materials suffer from insufficient creep resistance at the intended service temperature of about 700&#186;C. The present paper reports an experimental study of diffusion controlled deformation mechanisms in two-phase titanium aluminides which apparently cause the degradation of the strength properties at elevated temperatures. Electron microscope in situ heating studies were performed in order to characterize diffusion controlled dislocation climb. Climb velocities were analyzed in terms of diffusion coefficients and the critical vacancy supersaturation necessary for the operation of diffusion assisted dislocation sources. The experimental results will be discussed concerning the structural stability of two-phase titanium aluminides and potential factors for improving the high temperature strength.

Z6.29
IN SITU FOUR-POINT PROBE RESISTANCE MEASUREMENT DURING ANNEALING OF A PHASE SEPARATING THIN FILMS. T. Pennington and J.A. Barnard, Materials for Information Technology, The University of Alabama, Tuscaloosa, AL.

Understanding and modeling the underlying processes that produce resistance or resistivity in thin films are very complicated. As the electrons drift through the lattice, under the influence of an applied electric field, the resistivity is determined by the rate at which the electrons are scattered. The scattering centers in thin films are the same as in the bulk material (i.e. phonons, impurities, and defects) with the inclusion of film surface scattering and grain boundary scattering. In this work, a high vacuum annealing chamber for thin films has been built, with the incorporation of a high temperature four-point probe to measure the in situ changes in resistance of a phase separating co-sputtered binary alloy thin film with limited miscibility. The binary alloy thin film is composed of silver (Ag) solvent and cobalt (Co) solute. By comparing the changes in resistivity and the the time rate of change of the resistivity of a pure silver and cobalt thin film, with the changes in resistivity and the time rate of change of the resistivity of the binary alloy thin film, the rate of growth and size of the cobalt clusters can be determined in the binary alloy thin film. These resistivity measurements will also be compared with a computer model that is being developed concurrently with this work.

Z6.30
KINETICS AND MECHANISMS OF INTERMETALLIC GROWTH BY SURFACE INTERDIFFUSION. Yu. S. Kaganovskii, Dept. of Physics, Bar-Ilan University, Ramat-Gan, ISRAEL; L.N. Paritskaya, Dept. Physics of Crystals, Kharkov State University, Kharkov, UKRAINE.

The conditions of intermetallic formation on the surface and the kinetic regularities of growth and lateral spreading of an intermetallic layer along a free surface during surface interdiffusion in binary systems have been studied both theoretically and experimentally. A quasi-stationary solution for concentration distribution on the surface of growing intermetallics as well as kinetic equations of lateral spreading and profile evolution of growing layer have been obtained. The kinetic regularities of intermetallic formation on a free surface have been studied by means of optical microscopy, SEM and electron probe microanalysis, in experiments with two-layer diffusion couples A-B and with disperse particles A sintered to the surface B. The methods for determination of diffusion coefficients along intermetallic surface have been developed and applied to the systems Ni-Cd, Cu-Zn and Ni-Mo, with unipolar growth of $\gamma$-phases Ni₅Cd₂₁, Cu₅Zn₈ and Ni-Mo correspondingly.

SESSION Z7/EE5: JOINT SESSION:
POINT DEFECTS AND DIFFUSION IN Si AND SiGe
Chair: Nick E.B. Cowern
Wednesday Morning, April 15, 1998
Golden Gate C1

8:30 AM *Z7.1/EE5.1
ATOMISTIC SIMULATION STUDIES OF ION IMPLANTATION AND DOPANT DIFFUSION IN SILICON. Tomas Diaz La Rubia, Maria Caturla, M.D. Johnson and J. Zhu, Lawrence Levermore National Laboratory, Livermore, CA.

We discuss the development and application of a first-principles-based atomistic simulation methodology to model ion implantation and dopant diffusion in silicon. The method is based on a Monte Carlo approach where defects and dopants resulting from the implant step are allowed to diffuse at high temperature according to probabilitites based on their respective diffusivities and concentrations. The entropies and activation energies for diffusivity are calculated via first principles ab intio total energy methods. We include Fermi level and concentration-dependent diffusion effect by taking into account the charge state of the various defects and dopants. We present results for boron implantation and anneals in silicon at implant energies from 1 keV to 80 keV, and annealing temperatures from 700 C to 1100C. In addition, we discuss the effect of ramp rate during the anneal. We present results on the time dependence of transient enhanced diffusion and of the substitutional boron fraction during the anneal.
Work carried out under the auspices of the US Department of Energy by Lawrence Livermore National Laboratory under contract W-7405-Eng-48

9:00 AM *Z7.2/EE5.2
BORIDATION-ENHANCED-DIFFUSION OF BORON. Aditya Agarwal, D.J. Eaglesham, H.-J. Gossmann, S.B. Herner, L. Pelaz, and D.C. Jacobson, Bell Laboratories, Lucent Technologies, Murray Hill, NJ; T.E. Haynes, Solid State Division, Oak Ridge National Laboratory, Oak Ridge, TN;.

We show that formation of a silicon-boride phase at the silicon surface enhances boron diffusion in the bulk. This phenomenon, which we term ${\it Boridation-Enhanced-Diffusion (BED)}$, has serious implications for ultra-shallow junction processing, since the threshold boron concentration for formation of a silicon boride phase can be exceeded by boron implantation at ultra-low energies. BED from a variety of boron implantation and deposition conditions has been quantified using the method of embedded diffusion markers, and the silicon boride phase has been investigated by electron microscopy. We will discuss these data, possible mechanisms for the diffusion enhancement, and the implied constraints on shallow junction processing.

9:30 AM *Z7.3/EE5.3
IMPURITY DIFFUSION IN SiGi ALLOYS: STRAIN AND COMPOSITION EFFECT. Arne Nylandsted Larsen, Institute of Physics and Astronomy, University of Aarhus, Aarhus, DENMARK.

The advent of epitaxial-growth techniques such as molecular-beam epitaxy (MBE) or chemical-vapour deposition (CVD) dedicated to the growth of epitaxial layers of group-IV semiconductors and their alloys, has opened up new possibilities for novel types of diffusion experiments and thus, for critical tests of existing diffusion theories. Test structures for diffusion can now be grown consisting of well-defined narrow distributions of the tracer impurity in epitaxial layers of bi-axial strained or relaxed alloys of different compositions. This enables studies of alloy effects and/or strain effects on diffusion with varying strain energy for a well-defined type of strain (tensile or compressive). In this talk, we will present a review on recently published results on impurity diffusion in strained and relaxed, epitaxial SiGe alloy layers. Most of these studies have focused on the diffusion of boron and antimony as representatives of interstitialcy and vacancy assisted diffuser, respectively. The effect of biaxial strain on the enthalpies of migration and diffusion of the vacancy and the self-interstial as deduced from these experiments will be discussed.

10:30 AM Z7.4/EE5.4
MICROSCOPIC PROCESSES OF DAMAGE PRODUCTION DURING ION IMPLANTATION STUDIED BY COMBINING TIME-ORDERED BCA WITH MD SIMULATIONS. Matthias Posselt, Forschungszentrum Rossendorf, Institute of Ion Beam Physics and Materials Research, Dresden, GERMANY.

Ion bombardment causes collision cascades which form the nuclei of the implantation damage. After a single ion impact, the temporal development of such regions is characterized by ballistic, athermal, rapid thermal, and thermally activated processes. The time scale, the type of interaction as well as the common methods of atomistic modeling are very different for the respective processes. Therefore, a microscopic description of the entire damage history is difficult. Ballistic processes are well described by computer simulations based on the binary collision approximation (BCA) which allow an accurate and efficient prediction of ion range profiles. However, phenomenological models must be used to consider damage accumulation since the formation of the as-implanted defect structure is determined by athermal, rapid thermal and thermally activated processes which are beyond the scope of a BCA code. Classical molecular dynamics (MD) simulations of low-energy single ion impacts have led to considerable progress in understanding these processes on the atomistic level. Unfortunately, MD is a very CPU-intensive technique. It is therefore limited to special fundamental studies. The present work proposes a combination of BCA and MD simulations in order to investigate microscopic processes of damage production under more realistic conditions and in a more efficient manner. Since common BCA codes do not treat the time dependence of ballistic processes a new time-ordered program has been developed. It is suitable to study the temporal evolution of collision cascades produced by ions of common implantation energies. The code is applied to follow the motion of the impinging ion and the recoil atoms until their energy becomes lower than a certain threshold which must exceed the limit of about 100 eV below that the BCA is no longer valid. When the particles are stopped, time, position and velocity are stored. These values are used to restart the particle motion in subsequent MD simulations of the athermal and rapid thermal processes. Particular attention has been paid to the interface between the two types of simulations. The MD simulations are performed in representative regions of the collision cascade which are generally much smaller than the entire cascade volume. For characteristic examples the creation of the as-implanted defect structure is analyzed in dependence on the position of the MD box, on the energy and the mass of the ion, and on the target temperature.

10:45 AM Z7.5/EE5.5
NEW TIGHT-BINDING METHOD FOR SIMULATION OF DEFECT CONFIGURATIONS, CREATION AND DIFFUSION MECHANISMS IN SOLIDS: APPLICATION TO SILICON. Z.M.Khakimov, Inst of Nuclear Physics, Dept of Modelling of Physical and Chemical Processes, Tashkent, UZBEKISTAN.

The empirical and semiempirical tight-binding approaches are now in wide use in materials science. However they still have been developing separately for structural and electronic properties. Meanwhile, reliable prediction of materials properties and processes in them under influence of different external fields (heating, tension, irradiation, etc.) requires the structural and electronic properties to be considered in the framework of the same approach and with comparable accuracy, for instance, to involve charge state effects in state and motion of point defects in semiconductors and isolators.

This report presents the tight-binding method of new generation (and its realization for molecular dynamics simulation) which is free from the abovementioned disadvantages of existing tight-binding methods. This method based on the new total energy expression consisting of well-shared and self-reduced terms allowing us to calculate configurational and spectroscopic energies of defects accurately enough in the same computational scheme.

The accuracy and reliability of the method will be demonstrated considering defect state problems in c-Si and a- Si (electron-enhanced-atomic diffusion, metastable defect creation, defects with effective-negative correlation energies), and small clusters of C, Si and SiC (stable geometries, potential ionizations and electronic affinities, etc.) and comparing obtained results with available ab initio calculation results and experimental data.

11:00 AM Z7.6/EE5.6
CRYSTALLINE STRUCTURE AROUND THE SINGLE VACANCY IN SILICON: FORMATION VOLUME AND STRESS EFFECTS. A. Antonelli, Unicamp, Inst de Fisica, Campinas, BRAZIL; E. Kaxiras, Harvard Univ, Dept of Physics and Div of Applied Sciences, Cambridge, MA; D.J. Chadi, NEC Research Inst, Princeton, NJ.

The atomic structure surrounding the isolated neutral vacancy in silicon was examined by first-principles total energy calculations in the framework of local-density theory using norm-conserving pseudopotentials. The calculations were done mostly on a 64-atom supercell containing one vacancy per cell and with a 216-atom supercell to investigate finite size effects. Our results indicate the existence of two competing distortions of the crystalline lattice around a vacancy. The first type of distortion is strongly asymmetrical, enhancing the pairing of atoms, whereas the second type is much more symmetrical with practically no pairing of atoms. Both distortions produce large relaxation volumes and, at zero pressure, both have essentially the same formation energy. The results of our calculations indicate that the energy barrier in going from the symmetric to the asymmetric distortion is very small, of the order of a few meV. Therefore, under no external pressure, both types of structures should occur in equivalent concentrations. However, since the two structures have distinct formation volumes, the relative contribution of each distortion can be changed by the application of stress. We provide specific predictions of pressure effects on the concentration of the two types of vacancy structures.

11:15 AM Z7.7/EE5.7
ACTIVATION ENTHALPY OF SB DIFFUSION IN BIAXIALLY COMPRESSED SiGe LAYERS. A.Yu. Kuznetsov, J. Cardenas, B.G. Svensson, Royal Institute of Technology, Solid State Electronics, Kista-Stockholm, SWEDEN; A. Nylandsted Larsen, J. Lundsgaard Hansen, Institute of Physics and Astronomy, University of Arhus, DENMARK.

Diffusion in strained and relaxed Si /Si_1-xGe_x heterostructures is currently a topic of great interest that serves as a nice example for how biaxial strain actually affects the diffusion mechanisms in Si-based materials. In the present contribution the effect of the enhancement of Sb diffusion in compressively strained Si_1-xGe_x layers (x = 0.1, 0.2 and 0.3) is investigated. Sb diffusion coefficient in compressively strained Si_1-xGe_x (D^*_Sb) are extracted experimentally and subsequently compared with those in relaxed Si_1-xGe_x of the same composition (D_Sb). The contribution of biaxial compression ($\Delta\1\Omega\1^*_{Sb}$) to the activation enthalpy of Sb diffusion in strained Si_1-xGe_x ($\Delta$H^*_Sb) is estimated by the ratio of D^*_Sb/D_Sb which yields values of $\Delta\1\Omega\1^*_{Sb}$ of (0.42$\pm$0.30), (1.10$\pm$0.30) and (1.86$\pm$0.6)eV for x=0.1, 0.2 and 0.3, respectively. It is shown that the experimental values of $\Delta$H^*_Sb = $\Delta$E_Sb - $\Delta\1\Omega\1^*_{Sb}$ are in agreement with the theoretical prediction which equals $\Delta$E^*_Sb-$\sigma\1\Delta$V^*_Sb, with the activation volume $\Delta$V^*_Sb = $\Omega$, where $\Omega$ is the volume of the silicon lattice site, $\Delta$E^*_Sb is the activation energy of Sb diffusion in relaxed Si_1-xGe_x and $\sigma$ is the actual biaxial compression.

11:30 AM Z7.8/EE5.8
PROPERTIES OF VACANCIES IN SILICON DETERMINED BY OUT-DIFFUSION OF ZINC FROM SILICON. Axel Giese¹, Hartmut Bracht^2,3, Jack T. Walton² and Nicolaas A. Stolwijk¹, ¹ University of Müenster, Institut für Metallforschung, Müenster, Germany; ² Lawrence Berkeley National Laboratory and ³ University of California at Berkeley, Berkeley, CA.

We report the first out-diffusion experiments of Zn in dislocation-free homogeneously Zn-doped Si monocrystals. Diffusion of Zn in Si is known to be mediated by interstitial-substitutional diffusion mechanisms, i.e. the kick-out mechanism ($\mbox{Zn}_i \rightleftharpoons \mbox{Zn}_s + I$) and the dissociative mechanism ($\mbox{Zn}_i + V \rightleftharpoons \mbox{Zn}_s$). These reactions account for a changeover of interstitially dissolved Zn_i to the substitutional position Zn_s which involves Si self-interstitials I or vacancies V. Previously performed experiments with Zn into Si have shown that Zn in-diffusion is well described by the kick-out model yielding data for I-related properties like the thermal equilibrium concentration C_I^eq and the diffusivity D_I of Si self-interstitials.Compared to Zn in-diffusion, out-diffusion of Zn from homogeneously Zn-doped Si is expected to be mainly governed by the dissociative mechanism. We have performed out-diffusion experiments of Zn in closed quartz-ampoules for different Zn partial pressures. Out-diffusion profiles of Zn measured with the aid of the spreading-resistance technique are accurately described by interstitial-substitutional exchange mechanisms. Taking into account kick-out model parameters extracted from previous Zn in-diffusion profiles, fitting of our experimental profiles yields data for the equilibrium concentration C_V^eq and the diffusivity D_V of vacancies in Si. Our results are considered to be more reliable than literature data for C_V^eq and D_V deduced from metal in-diffusion experiments in Si.

11:45 AM Z7.9/EE5.9
THE DEFECT EVOLUTION AND THE ANNEALING BEHAVIOR OF VACANCY-LIKE DEFECTS IN SILICON AFTER ION IMPLANTATION. Stefan Eichler, Frank Börner, Reinhard Krause-Rehberg, Fachbereich Physik der Martin-Luther-Universität Halle-Wittenberg, Halle(Saale), GERMANY.

The fluence-dependent evolution and the annealing behavior of vacancy-like defects in Si(100) implanted at room temperature with boron, silicon, and arsenic ions was measured by Variable-Energy Positron Annihilation spectroscopy (VEPAS). The defect investigations were accompanied by Rutherford Backscattering Spectroscopy (RBS), Infrared Absorption Spectroscopy (IR), and Raman spectroscopy (RS). In addition, Monte-Carlo calculations were done with the Transport of Ions in Matter (TRIM) code. The samples were implanted with different fluences (over two orders of magnitude) for each ion species and we have tried to produce a nearly homogenous defect distributions using a multiple implantation mode with 3 energies. The resulting defect distributions up to a depth of about 1 $\mu$m found by TRIM simulations as well as by the methods VEPAS and RBS are in good accordance. In general, both of these integral techniques show a square-root dependence of the defect density on the fluence. This dependence is in an agreement with the model of defect nucleation by Chadderton. A deviation from this rule is detected only for the silicon self-implantation due to an increased recombination of self-interstitials and silicon vacancies during the implantation process. The annealing behavior of implantation induced vacancy-like defects is dependent on the defect concentration in the as-implanted state. The recrystalization of amorphous zones or pockets, the agglomeration and recombination of defects, and the formation of defect complexes were observed during successive isochronal annealing.

SESSION Z8: DIFFUSION IN SEMICONDUCTORS
Chairs: Nick E.B. Cowern and Ulrich M. Goesele
Wednesday Afternoon, April 15, 1998
Golden Gate C1

1:30 PM *Z8.1
THE FERMI-LEVEL EFFECT AND MECHANISMS OF GROUP III SUBLATTICE ATOM DIFFUSION IN III-V COMPOUND SEMICONDUCTORS. Teh Yu Tan, Chang-Ho Chen, Ulrich Goesele, Department of Mechanical Engineering and Materials Science, Duke University, Durham, NC.

For elements on the group III sublattice of GaAs and/or related III-V compounds, It has been determined in recent years that effects of the Fermi-level and non-equilibrium point defects play the governing role in their diffusion processes. The triply-negatively-charged group III vacancies govern the group III element diffusion and the diffusion of Si donor atoms in GaAs and GaAs/AlAs type superlattices under intrinsic and n-doping conditions. The concentration of these vacancies can exhibit a negative temperature dependence in highly n-doped GaAs in that it will increase as the temperature is lowered. In lightly n-doped superlattices, an additional consideration of the electrical effect due to bandgap inhomogeneities and/or hetero-junctions has allowed quantitative fits of experimental results and thus refute recent doubts on the validity of the Fermi-level effect model itself and on whether the triply-negatively-charged group III vacancy is indeed the dominating defect. Under sufficiently strong p-doping conditions, the doubly-positively-charged group III element self-interstitials play the governing role for the diffusion of group III elements and the acceptor atoms Zn and Be. Indiffusion of Zn to a high concentration into GaAs under a high As vapor pressure leads to kink-and-tail type of diffusion profiles because of the action of two opposing thermodynamic driving forces for producing two limiting GaAs crystal compositions in one specimen via Zn indiffusion induced Ga self-interstitials.

2:00 PM *Z8.2
SELF-DIFFUSION IN ISOTOPICALLY CONTROLLED HETEROSTRUCTURES OF ELEMENTAL AND COMPOUND SEMICONDUCTORS. Hartmut Bracht and Eugene E. Haller, Lawrence Berkeley National Laboratory, University of California at Berkeley, Berkeley, CA; Karl Eberl and Manuel Cardona, MPI für Festköerperforschung, Stuttgart, GERMANY; Robert Clark-Phelps, Charles Evans and Associates, Redwood City, CA.

We report the first direct measurements of the Si and Ga self-diffusion coefficients in isotopically controlled multilayer structures of Si and Al_xGa_1-xAs, respectively. The latter structures were also used to determine Al-Ga interdiffusion. Diffusion and mass transport of atoms occupying lattice sites in a crystalline solid are governed by native defects including vacancies (V), self-interstitials (I) and, in compounds, also antisite defects, with self-diffusion being the most fundamental process. To study self-diffusion, the atoms have to be labeled, e.g., by using radioactive isotopes. However, strong experimental restrictions arise when the radioactive tracer has a short half-life, or when the deposition of the tracer causes unknown boundary conditions at the surface. These experimental problems can be avoided by studying self-diffusion in isotopically controlled heterostructures. We have performed Si self-diffusion experiments between 850 and 1400&#186;C with highly enriched ²⁸Si layers grown by chemical vapor deposition on natural Si substrates. Diffusion profiles of ³⁰Si were measured with high resolution secondary ion mass spectrometry. The temperature dependence of Si-tracer self-diffusion coefficients extracted from the ³⁰Si profiles shows whether Si self-diffusion is dominated by Si interstitials above and by Si vacancies below a critical temperature. Such a change in the mechanism of self-diffusion has been assumed since both I and V may contribute to Si self-diffusion. We also will present extensive self- and interdiffusion experiments with buried Al⁷¹GaAs/Al⁶⁹GaAs/⁷¹GaAs isotope heterostructures grown by molecular beam epitaxy. This layer ordering is especially advantageous for the determination of not only Al-Ga interdiffusion, but also Ga self-diffusion in AlGaAs.

2:30 PM *Z8.3
ARSENIC DIFFUSION IN INTRINSIC AND HEAVILY ZINC-DOPED GALLIUM ARSENIDE. Guido Bösker, Nicolaas Stolwijk, Helmut Mehrer, Inst f Metallforschung, Univ Münster, Münster, GERMANY; Angela Burchard, ISOLDE Collaboration, CERN, Geneva, SWITZERLAND; Ulf Södervall, Dept of Physics, Chalmers Univ of Technology, Göteborg, SWEDEN.

Whereas in recent years quite some data related to point defects on the Ga sublattice have been obtained from diffusion experiments under intrinsic, n-type and p-type doping conditions there is a severe lack of information regarding the As sublattice. In particular, this is true for As diffusion for which the two limited groups of measurements reported in the literature are not mutually consistent. We have performed self-diffusion experiments on intrinsic GaAs and -for the first time- on p-type GaAs with different zinc doping levels ($1\times 10^{19} - 3\times 10^{19}\, \mbox{cm}^{-3}$). The radioactive isotope ⁷⁶As was produced by neutron activation and contained in a vapor-phase diffusion source in order to study the self-diffusion behavior at elevated temperatures. Another radio-isotope, ⁷³As, was implanted at the ISOLDE, a facility of CERN, and preferentially used at lower temperatures because of its longer half-life time. Tracer penetration profiles were determined by serial sectioning with the aid of Ar-ion beam sputtering followed by measuring of the radioactivity of each section. Appropriate fitting to the measured profiles yields As diffusion coefficients in accessible temperature ranges which vary for the different doping levels investigated. Outdiffusion of Zn dopant atoms from the surface region was checked by SIMS analysis after diffusion annealing and taken into account in the evaluation of the As profiles. The As diffusivity in extrinsically Zn-doped crystals is found to be markedly enhanced compared to intrinsic GaAs. This shows that As utilizes positively charged intrinsic defects as diffusion vehicles. The present results globally comply with theoretical calculations which predict 1+ to 3+ charge states for both As vacancies and As interstitials depending on the type and degree of doping.

3:30 PM Z8.4
EVIDENCE FOR NON-EQUILIBRIUM VACANCY CONCENTRATIONS CONTROLLING INTERDIFFUSION IN III-V MATERIALS. W.P. Gillin, Department of Physics, Queen Mary and Westfield College, University of London, London, UNITED KINGDOM; O.M. Khreis and K.P. Homewood, School of Electronic Engineering, Information Technology and Mathematics, University of Surrey, Guildford, Surrey, UNITED KINGDOM.

Interdiffusion and self-diffusion in III-V semiconductors has usually been assumed to operate through the diffusion of point defects, the concentration of which are at thermal equilibrium. We have studied the interdiffusion of multiple quantum well structures, grown under a range of growth conditions, which contain a thin source of vacancies. This enables simultanious measurement of the interdiffusion coefficient, the diffusion coeffcient for vacancies and the background concentration of vacancies in a single experiment. We have shown that the interdiffusion at all temperatures is governed by a constant background of vacancies in the material. This result shows that the vacancy concentration in GaAs is not at a thermal equilibrium value, as has been widely assumed. Rather it has a value which is frozen in, probably at the GaAs crystal growth temperature.

3:45 PM Z8.5
TIME DEPENDENCE OF PRECIPITATE SIZE DISTRIBUTION IN LOW TEMPERATURE GaAs. C.-Y. Hung, J.S. Harris, Jr., A.F. Marshall*, and R.A. Kiehl, Solid State Electronics Laboratory, Stanford University, Stanford, CA; *Center for Materials Research, Stanford University, Stanford, CA.

We are studying a self-assembly technique to the fabrication of single-electron tunneling devices in which the arsenic particles serve as small metallic islands separated by GaAs tunneling barrier. Nanometer-scale arsenic particles can be formed in GaAs by molecular beam epitaxy at low temperatures and subsequent high temperature annealing. The average precipitate size can be designed to be a few nanometers to tens of nanometers by adjusting the annealing temperature. While most work on arsenic precipitation has focused on large particles a few tens of nanometer in diameter, the single-electron applications would require particles approaching one nanometer in diameter with highly uniform size distribution. In the present work, we investigated the time dependence of the size distribution of arsenic precipitates during annealing for both large and small size regimes. The size distribution changes with time due to the diffusion of atomic arsenic in the lattice. A narrowing of the size distribution of arsenic precipitates in non-stoichiometric GaAs was observed during annealing for precipitates in the small regime of a few nanometers. This improvement in size uniformity is in marked contrast to what is observed for large precipitates, which coarsen with a widening distribution similar to that of classical Ostwald ripening. The difference between large and small precipitates were also examined by high resolution transmission electron microscopy. A possible mechanism for this interesting and potentially useful behavior will be presented.

4:00 PM Z8.6
THE ANNEALING EFFECTS ON ZNCDSE/ ZNSE QUANTUM WELLS AND ZNSE/GAAS INTERFACES. Ru-Chin Tu, Yan-Kun Su, and Shoou-Jinn Chang, Department of Electrical Engineering, National Cheng Kung University, Tainan, Taiwan, CHINA.

The disordering of ZnCdSe/ZnSe quantum wells and the outdiffusion of Ga atoms from the GaAs substrate initiated by thermal annealing were confirmed by photoluminescence (PL) spectroscopy and secondary ion mass spectrometry (SIMS). The 11K PL main peak at 2.67eV disappeared when the samples were annealed above 550*C, while two extra PL peaks at 2.0 and 2.3eV respectively were observed when samples were annealed above 500*C. SIMS results showed the interdiffusion of Cd in the ZnCdSe/ZnSe quantum well regions and the strong outdiffusion of Ga atoms from the substrate into the epilayer side. The diminish of main PL peak was attributed to the Cd interdiffusion of ZnCdSe wells and the outdiffusion of Ga atoms into the well regions. The emergence of the two extra PL peaks after sample annealing indicated the formation of Ga related complexes and/or defects due to the Ga outdiffusion into the epilayer side, which, in addition to the Cd interdiffusion, may be responsible for the relatively short lifetime of ZnSe-based laser diodes.

4:15 PM Z8.7
THRESHOLD CHARACTER OF Zn DIFFUSION IN InP. Alexander Kamanin, Yury Kudryavtsev, Natalie Shmidt, Ioffe Physico-Technical Institute, St.Petersburg, RUSSIA.

The initial diffusion stage (IDS) of Zn in InP from polymer spin-on films has been investigated. IDS is an establishment stage at which temperature of the system rises from room temperature to final temperature T_f corresponding to temperature of isothermal diffusion. In the experiment, the isothermal diffusion stage was eliminated. Thus IDS was immediately followed by cooling down the system. IDS was investigated in the final temperature range from 360&#186;C to 650C. As this took place, heating duration (IDS time) was fixed and was 180 seconds. The heating rate was variable. Distribution profiles of both impurity and host atoms were determined by secondary-ion mass-spectrometry (SIMS). A carrier concentration was measured from C-V characteristics. It was found that no Zn diffusion was detected by SIMS at T_f up to 365&#186;C. At the same time the ultra-fast diffusion took place at T_f of 375&#186;C and above. The process was characterized by a high concentration and a deep penetration of Zn atoms and by a low acceptor concentration. The Zn atom concentration at $T_f=375^{\circ}$C was obtained by SIMS to be more than 10¹⁸ cm^-3, while the acceptor concentration was 10¹⁶ cm^-3 with a p-n-junction position of 0.8 $\mu$m from the substrate surface. Effective diffusivity was estimated to be at least three orders of magnitude more than the one reported by other authors for Zn interstitials under the isothermal conditions. Because the conventional diffusion models are restricted to the isothermal case of diffusion, they fail to describe IDS, especially the threshold diffusion temperature.

4:30 PM Z8.8
OUT DIFFUSION OF OXYGEN IN CZOCHRALSKI SILICON AT LOW TEMPERATURES. S.A. McQuaid, B.K. Johnson, MEMC Electronic Materials, St. Peters, MO; D. Gambaro, R. Falster, MEMC Electronic Materials SpA, Novara, ITALY.

While values of diffusion coefficient of isolated oxygen atoms derived from measurements of jump frequencies at low temperatures (T<350C) are close to an extrapolation of values determined from out-diffusion profiles following high temperature (T>700C) treatments, reports indicate that rates of out-diffusion at temperatures below 700C lead to estimates which deviate from the expected values. Effective diffusion coefficients, derived by fitting an error function to the measured near-surface concentration profile have been reported to be up to seven orders of magnitude greater than expected. These values for anneals at 450C decreased with anneal time. The inferred depth scale over which the concentration varied appears to be independent of time. We present independent measurements of out-diffusion profiles measured by SIMS. While the surface concentration remains essentially independent of anneal time as expected at high temperatures (>700C), it does not generally do so during anneals at lower temperatures. At 450C the depth scale of the profile is independent of time, as implied by previously, but the surface concentration decreases. The magnitude of the decrease correlates with independent measurements of the loss of oxygen atoms from interstitial sites in the bulk. Therefore, simple diffusion described by the error function is not taking place and a new model is required. The essential features of the proposed model are a) slow transformation of isolated O- atoms, b) rapid diffusion of the resultant defect and c) competitive trapping on the surface and within the bulk. An exponential fit to the near-surface concentration profiles, leads to estimates of the transformation rate and the bulk trapping rate. The former is close to the expected rate of interaction of randomly diffusing O-atoms in a dilute solution while the latter is close to a previously deduced dissociation rate of a complex responsible for in-diffusion from an implanted layer.

SESSION Z9: POSTER SESSION: DIFFUSION IN SEMICONDUCTORS AND IONIC SOLIDS
Chairs: C.R.A. Catlow and Nick E.B. Cowern
Wednesday Evening, April 15, 1998
8:00 P.M.
Salon 7

Z9.1
KINETIC MONTE CARLO SIMULATION OF DIAMOND FILM GROWTH WITH THE INCLUSION OF SURFACE MIGRATION. Armando Netto, Michael Frenklach, University of California, Department of Mechanical Engineering, Berkeley, CA.

Diamond films are of interest to many practical applications but the technology of producing high-quality and low-cost diamond still lacking. To reach this goal, it is necessary to understand the mechanism underlying diamond deposition. Most reaction models advanced thus far do not consider surface diffusion, and modeling based on such models does not reproduce correctly the morphological features of diamond films. Our recent theoretical results, founded on quantum-mechanical calculations and localized kinetic analysis, suggested the critical role that surface migration must play in growth of diamond films. In this paper we report three-dimensional time-dependent Monte Carlo simulations of diamond growth which consider adsorption, desorption, lattice incorporation, and surface migration. The reaction mechanism includes seven gas-surface, four surface migration, and two surface-dimer reaction steps. The reaction probabilities are founded on the results of quantum-chemical and transition-state-theory calculations. The kinetic Monte Carlo simulations showed that, starting with an ideal 100-(2x1) reconstructed surface, the model is able to produce a continuous film growth. The smoothness of the growing film and the developing morphology are shown to be influenced by rate parameter values and hence by deposition conditions, such as temperature and gaseous species concentrations.

Z9.2
LIGHT SCATTERING MEASUREMENTS OF THE SMOOTHING OF TEXTURED GAAS AND INP IN MOLECULAR BEAM EPITAXY. A. Ballestad, M. Adamcyk, D. Grandmaison, T. Pinnington and T. Tiedje*, Advanced Material Processing and Engineering Laboratories, Department of Physics and Astronomy, *also Department of Electrical and Computer Engineering, University of British Columbia, Vancouver, B.C., CANADA.

Epitaxial regrowth on periodically textured surfaces is a critical step in the fabrication of single mode semiconductor lasers. When heating textured semiconductor substrates prior to regrowth, the surface texture can be lost due to lateral transport of material caused by surface diffusion. The lateral transport process which smooths the surface is not well understood but is related to the kinetic roughening which takes place during growth on initially smooth surfaces. In this work, we use in situ laser light scattering [1] to study the time-dependence of the structure of microfabricated 1D gratings on GaAs and InP substrates, during heating and subsequent epitaxial growth by molecular beam epitaxy (MBE). One dimensional gratings with pitches between 400 and 1200 nm were fabricated using electron-beam lithography with PMMA photoresist, followed by plasma etching with CH₄/H₂ or by wet-etching in dilute H₂SO₄:H₂O₂:H₂O. The grating pitch determines the diffraction angle of 488 nm laser light incident on the surface. By writing grating patterns with different pitches in the same 2 mm² region on the wafer, we are able to simultaneously monitor the diffracted intensity from two or three different periodicity gratings, through different optical ports in the MBE chamber. The diffracted intensity is related to the amplitude of the surface grating. After an initial transient the intensity of the diffracted light decreases exponentially with time during growth. This behavior is consistent with the time dependence of the smoothing expected from the noise-free Edwards Wilkinson
(EW) equation, [2]:

where h(r,t) is the surface height at position r and $\nu$ is a temperature dependent constant, and the microfabricated surface texture acts as the initial condition. The observed intensity of the diffracted light is compared with a simulation in which the smoothing is described by the EW equation and the scattered light intensity is determined using an electromagnetic model for the scattering from the simulated surface texture. Ex-situ SEM measurements on cleaved cross sections are used to investigate the surface texture on patterned substrates removed from the growth chamber at various stages during the temperature ramp-up and subsequent re-growth. From these measurements and from the light scattering results we conclude that at 600 C the textured GaAs surface develops facets and then smooths slowly before more rapid smoothing takes place during growth. For growth at 1 micron/hr we find that $\nu$ is 2x10^-13 cm²/s, in agreement with earlier results for smoothing of random surfaces [3]. Additional experimental results on the spatial frequency and temperature dependence of the smoothing will be presented for both GaAs and InP. 1. T. Pinnington et al., Phys. Rev. Lett. 79, 1698 (1997).
2. A. L. Barabasi, H. E. Stanley, Fractal Concepts in Surface Growth, Cambridge UP (1995).
3. C. Lavoie, PhD Thesis, UBC (1995).

Z9.3
SPACE CHARGE EFFECTS ON DOPANT DIFFUSION COEFFICIENT MEASUREMENTS IN SEMICONDUCTORS. Konstantin Gartsman, David Cahen, Weizmann Institute of Science, Rehovot, ISRAEL; Igor Lubomirsky, UCLA, Department of Electronic Engineering, Los Angeles, CA.

When interested in diffusion of dopants in semiconductors, the material can usefully be considered as a mixed ionic/electronic conductor with very low ionic transfer number. In the indirect methods of diffusion coefficient measurements that are used, systematic errors are likely to affect the results. The highest contribution to these errors is introduced by the presence of an internal electric field, i.e., by space charge effects. The electric field can be the result either of a dopant concentration gradient or of external bias, applied during the measurement. We consider three methods: p-n junction motion, the time dependence of capacitance under reverse bias (transient ion drift, TID), and current or potential decay under constant potential or current. Having derived the possible large acceleration of diffusion that can result from drift (up to a few orders of magnitude), we examine critically the three methods, their open and hidden assumptions and their range of applicability. The p-n junction motion method is valid only if space charge effects can be neglected under the conditions of the measurement. It is not applicable if dopant density is more than intrinsic carrier density, but is very useful at sufficiently high temperatures. The potentiostatic decay method is not applicable for low conducting and heavily compensated materials. However, it gives correct results for degenerated or strongly doped materials. In contrast to it, TID has the best performance for materials that are far from degenerate, but will give erroneous results when used for heavily compensated materials. We use the results of our analysis to review experimental data obtained by different direct and indirect methods for Cu diffusion in CuInSe2, an issue of considerable current interest for solar cells and compare them to our recent direct (tracer) data.

Z9.4
STUDIES ON NITROGEN ION IMPLANTED CADMIUM SELENIDE THIN FILMS. C. Sanjeeviraja, R. Chandramohan, S. Rajendran, T. Mahalingam, Physics Department, Alagappa University, Karaikudi, INDIA; M. Jayachandran, Central Electrochemical Research Institute, Karaikudi, INDIA.

Cadmium selenide is an interesting material of the II-VI family of semiconductors which is used in the fabrication of thin film transistors, low cost solar cells, etc. The solving of doping puzzles in II-VI semiconductors with group V elements like N, P are of considerable interest recently. Electrodeposition is one of the cost effective methods for the synthesis of polycrystalline thin films. In this study, the synthesis of CdSe thin films from aqueous acidic solution bath onto tin oxide coated conduction glass plates are reported. The implantation of 100 KeV N₂ ion onto the electrosynthesized CdSe thin films by employing different dosages are described. The effect of N₂ ion implantation on the structural, electrical and optical properties are discussed. A marked decrease in the resistance of the films from 200 ohms to 10 ohms are observed. The defective nature of the implanted sample and removal of these defects on subsequent annealing are also dealt. A change from `n' type to `p' type is also observed. This is attributed to the occupation of metallic sites in the vicinity of the surface by the nitrogen ions and forming a cluster. The retrieval of the conductivity on annaling is attributed to the removal of clusters and subsequent movement to the interstitials. The results are discussed in details.

Z9.5
THE EFFECT OF HIGH TIN-BACKGROUND DOPING ON THE DIFFUSION OF ANTIMONY IN SILICON. Jacob Fage-Pedersen, Peter Gaiduk, John Lundsgaard Hansen, Per Kringhøj, Arne Nylandsted Larsen, Institute of Physics and Astronomy, Univ of Aarhus, DENMARK; Margareta Linnarson, Royal Inst of Technology, Solid State Electronics, Kista-Stockholm, SWEDEN.

The diffusion of dopants in Si at high concentrations is still a controversial topic. We have previously demonstrated that the diffusion of Sb and As in Si in a uniform P background doping is strongly enhanced for P concentrations higher than $2\times 10^{20}{\rm cm}^{-3}$. We have explained this behaviour as a collective phenomenon in which the vacancy, which is the diffusion vehicle, feels the presence of at least two donor impurities, and we have succesfully modelled the behaviour within the vacancy-percolation model. Other authors have explained this behaviour by a diffusion of more complicated impurity-vacancy complexes, or by a reduction of the activation energy for pair diffusion without assuming vacancy percolation. In this paper we present a study of Sb diffusion in Si as a function of Sn background doping. Sn is electrically neutral in Si and, as such, enables a diffusion study with no effect of Fermi level changes. At the same time Sn is known to trap vacancies, and a high concentration of Sn can therefore be expected to form a vacancy-percolation cluster in which the Sb impurity can diffuse. Since Sb is known to diffuse predominantly by the vacancy-assisted mechanism in Si it is a natural choice of tracer impurity for a study of vacancy assisted diffusion. The diffusion of Sb has thus been studied in MBE-grown Si samples with Sn background concentrations up to $3\times 10^{20}{\rm cm}^{-3}$ and low-concentration Sb-spikes. A significant enhancement in the Sb diffusion is observed for Sn concentrations above $\sim 2\times 10^{20}{\rm cm}^{-3}$. The upper limit of the investigated Sn concentration range has been set by TEM investigations showing that beyond these Sn concentrations precipitates form at the diffusion temperatures used, and thus inject point defects affecting the diffusion. The observed diffusivity enhancement is discussed in connection with the different models mentioned above.

Z9.6
QUANTITATIVE MEASUREMENT OF REDUCTION OF BORON DIFFUSION BY SUBSTITUTIONAL CARBON INCORPORATION. M.S. Carroll, L. D. Lanzerotti, J.C. Sturm, Dept. of Electrical Engineering, Princeton University, NJ.

Recently, dramatic suppression of boron diffusion due to both thermal diffusion and TED has been demonstrated through the incorporation of 0.5% substitutional carbon in the base of Si/SiGe/Si heterojunction transistor's (HBT)^[1]. In this talk we measure the effect of carbon on boron diffusion and TED through it's effect in the base of the HBT on the HBT's electrical properties. The HBT's are sensitive to boron diffusion lengths as small as 30Å  and therefore are more sensitive than SIMS for detection of boron outdiffusion. HBT's were grown by low temperature epitaxy and then annealed at high temperature or first subjected to ion implant damage near the surface region far from the boron and then annealed. Combined process and device modeling (TMA TSUPREM4 and MEDICI) are used to relate observed electrical characteristics (collector saturation currents and early voltage) to diffusion. A 50-75% reduction of the effective boron diffusivity at 855&#186;C is found from 0.5% substitutional carbon in the base without any implant damage. The interstitial concentration due to ion implant damage (which leads to TED) was reduced by over an order of magnitude at 647&#186;C using 0.5% substitutional carbon in the HBT SiGe bases both in regions with carbon and those nearby without carbon. This demonstrates that the carbon incorporation acts as an effective sink for excess interstitial.
^[1] Lanzerotti et. al. APL 70 (23), 9 June 1997)

Z9.7
ULTRA-FAST NISI₂ FORMATION IN P⁺-SI BY HIGH CURRENT DENSITIES. J.S. Huang and K.N. Tu, Department of Materials Science and Engineering, UCLA, Los Angeles, CA.

An ultra-fast lateral formation of epitaxial NiSi₂ line in p⁺-Si was created at room temperature under the application of current densities of 10⁶/cm². A NiSi₂ line of 1 $\mu$m wide and 140 $\mu$m long linking the cathode and anode contacts was formed within 1 second. Shorter lines can be formed with lower current densities. The shorter line formation was again completed within the 1 second and there was very little subsequent growth upon further stressing. The line length increased with applied current. The dependence of the current suggests that the line formation is limited by driving force rather than by kinetics. We propose that the formation is caused by electromigration as well as by heating due to electron-hole recombination.

Z9.8
GE DIFFUSION IN SNTE CRYSTAL. O.E. Kaportseva, T.B. Shatalova, V.B. Bobruiko, L.V. Yashina, V.F. Kozlovsky,V.I. Shtanov, Moscow State University, Chemistry Department, Moscow, RUSSIA.

Solid solutions on the base of A4B6 compounds are useful materials for IR-devises. Diffusion coefficients (D) are necessary for the optimization of doping processes. This report deals with the investigation of diffusion Ge in SnTe crystals. Diffusion study contained annealings of SnTe wafers with (100) surface in GeTe vapour, profile recording and D calculation. Thermodynamic and kinetic peculiarities of annealing should be taken into account because they can make strong influence on the resulting diffusion coefficients. To control P(GeTe) the temperature of GeTe vapour source must be less than sample temperature. Annealing conditions are determined by Pi-T-x-y phase diagram. P(Te2) also has to be controlled in order to keep initial deviation from stoichiometry in SnTe sample by an additional source of tellurium vapour. As for kinetic peculiarities under two-temperature annealing surface etching takes place due to sample re-sublimation in lower temperature zone. To minimise this process buffer gas is used. If its pressure is too high gas diffusion becomes the slowest stage of process. Regarding these two factors the optimal value of P(Ar) = 150Torr at 298 K was calculated. In this range the variations of Ar pressure do not effect on the diffusion coefficient and provide minimal re-sublimation. Depth profile analysis was carried out by X-ray difractometer and by electron probe X-ray microanalyzer Camebax, Microbeam JXA-840. Layer-by-layer X-ray difractometry allows to determine lattice constant changes connected with both concentration x and possible Te content changes. Microprobe analysis gives us concentration x profile. From these profiles Ge diffusion coefficients were calculated. It was found that D(Ge)= 3.8(4) 10^-10 cm2/s at 923K and vacancy mechanism is predominant. It is essential that Ge diffusion coefficient in SnTe crystal is higher than in PbTe crystal.

Z9.9
SPECIAL DIFFUSION MECHANISM IN SOLIDS, EQUILIBRIUM AND TRANSPORT PHENOMENA IN SUPERIONICS. Vladimir Koshkin, Polytechnic Univ, Physical Chemistry Dept, Kharkov, UKRAINE.

The model of short-living unstable pairs vacancy -interstitial (UP), being developed earlier (see [1]), is applied to the description of diffusion in semiconductors and high temperature metals. It is shown the participation of equilibrium UP together with Schottky's and Frenkel's defects permits to interpret paradoxes at temperature dependencies of diffusion coefficients as well as the unquenching of point defects for a number of solids. At a collaboration with Yu.Zabrodskii and Yu.Reshetnyak the model of equilibrium concentration of UP is used for the theory of superionic state taking into account the selfconsistent collective interaction which include UP and Frenkel's pairs as well. The phase transitions of both types and their temperature sequence as well as a transition to the ferroelectric state are described. The theory describes also the high frequency peculiarities of superionic crystals especially the strange wide band of absorption. The low frequency conductivity of superionic substances had been considered also with the accounting of ion - vacancy recombination which is an ion mobility limitation. The theoretical calculations are in agreement with the experimental results of Cava, Salamon, Prokhorov, Stefanovich et al. The UP seem to be a predominant type of equilibrium defects at least at the not too close packed solids.
[1] V.M.Koshkin, Yu.N.Dmitriev, Chemistry and Physics of Compounds with Loose Crystal Structure, Chemical Reviews, v.19, part 2, Harwood Academic Publishers, 1994, Switzerland, 138 p.

Z9.10
STUDY OF TRANSPORT AND ELECTRONIC PROPERTIES OF 1st-STAGE LITHIUM INTERCALATED GRAPHITE: A HARRIS FUNCTIOINAL APPROACH. K.R. Kganyago, P.E. Ngoepe, Materials Modelling Center, University of the North, Sovenga, SOUTH AFRICA.

One of the most important advances in recent years in the field of energy storage and conversion relates to the development of Li ion batteries based on lithium intercalated graphite anodes. First-principle total-energy calculations, based on the density functional theory, were carried out for a 1st-stage lithium intercalated graphite model. Molecular dynamics (MD) simulation have been extremely successful in describing the properties of systems of atoms for which the interatomic forces are of the van der Waals type and can be modelled in terms of pair interactions. In this study we apply an alternative approach to a combination of molecular dynamics and density functional theory (MD-DF) introduced by Car and Parrinello (CP), the Harris functional approach. This approach is based on a related energy functional that can be assigned a value for any density, specifically a superposition of site densities moving with the nuclear sites. In a normal cycle of a lithium battery, where LiC₆ is an anode, Li intercalate and de-intercalate into and from graphite respectively. The diffusion of lithium between the graphite layers is reported at different temperatures.

Z9.11
MOLECULAR DYNAMICS COMPUTER SIMULATIONS OF DIFFUSION IN SOLIDS. Phillip Pohl, Diana Fisler and Tina Nenoff, Sandia National Laboratories, Albuquerque, NM.

Computer simulations are used to help understand the diffusion of gases and cations in crystalline solids. Various zeolites are evaluated for separation of xylene isomers by molecular sieving membranes. The purpose of this work is to predict, using molecular modeling, which zeolite and in what composition is ideal for removing para-xylene from a mixture of a typical processing stream using these crystalline memebranes. The methods used assume that the permeating gases must adsorb into the pores and then transport via an activated diffusion mechnaism. Comparison of the predicted activation energies allows the optimization to take place. Results of the variation of cation type and composition will be presented.

Z9.12
AN OXYGEN POTENTIAL GRADIENT AS A POSSIBLE DIFFUSION DRIVING FORCE. B.A. Pint and K.B. Alexander, Oak Ridge National Laboratory, Metals and Ceramics Division, Oak Ridge, TN; O.R. Monteiro and I.G. Brown, Lawrence Berkeley National Laboratory, Berkeley, CA.

In high temperature oxidation, oxygen-active elements such as Y, Zr and Hf are added to alumina-forming alloys to improve the adhesion of the external $\alpha$-Al₂O₃ scale. During oxidation at 1000&#186;-1500C, many such elements are observed to diffuse from the alloy through the scale and nucleate oxide particles at the scale-gas interface. Once nucleated, the volume fraction of these particles increases with oxidation temperature and time. The continued increase in volume fraction with time suggests that diffusion is not merely driven by a concentration gradient of the oxygen-active elements. Pre-coating with a dense, 1$\mu$m-thick, plasma-deposited alumina layer prior to oxidation does not prevent these types of particles from nucleating and growing at the gas interface above the coating. The driving force for this diffusion phenomenon is attributed to the oxygen potential gradient across the metal-oxide-gas system and the high oxygen affinity of these elements.

Z9.13
IMPEDANCE OF INTER-DIFFUSION BETWEEN MGO AND YBCO BY SrTiO₃ BUFFER LAYER, Xingtian Cui, Metals & Ceramics Division, ORNL, Oak Ridge, TN; Q.Y. Chen, I. Rusakova, J.R. Liu, O. Minayeva and W.K. Chu, Texas Center for Superconductivity, University of Houston, Houston, TX.

Inter-diffusion of YBa₂Cu₃O$_{7-\delta}$ (YBCO) thin films grown on MgO substrate with and without a stain-relieved SrTiO₃ (STO) buffer layer has been investigated by Rutherford Backscattering Spectometry (RBS), and high resolution cross sectional transmission electron microscopy (XTEM). The in-situ growth of STO buffer layer along with the YBCO films was carried out by pulsed laser deposition. RBS and XTEM studies showed that the STO butter layers help to stop the copper of YBCO films diffusing into MgO substrates as well as to improve the crystalline quality of YBCO thin films.

Z9.14
CHROMIUM DIFFUSION IN SAPPHIRE CRYSTALS: INFLUENCE OF PREANNEALING CONDITIONS. E.G.Gontier-Moya, F. Moya, Laboratoire SERMEC, Faculte des Sciences de St Jérôme, Marseille, FRANCE; J. Bigarre, D. Juve, D. Treheux, Laboratoire Materiaux-Mecanique Physique, Ecole Centrale de Lyon, Ecully, FRANCE; C. Grattepain, Laboratoire de Physique des Solides, CNRS, Meudon, FRANCE.

Chromium diffusion coefficients in alumina single crystals were evaluated through penetration profiles determined by the SIMS technique. The samples were previously annealed at 1000ƒC (1 hour), 1500ƒC (4 hours), and 1700ƒC (24 hours). Diffusion experiments were carried out at 1400ƒC during 10 hours. The diffusion profiles presented two distinct parts, the former associated with bulk diffusion and the latter with dislocation diffusion. The results show that bulk diffusion coefficients are strongly dependant on annealing conditions (temperature and time): they decrease of about 2 orders of magnitude when the preannealing temperature was increased from 1000ƒC to 1700ƒC. These results are discussed taking into account the dislocations and point defects in alumina. We consider the role played by the linear defects resulting from surface polishing, as well as extrinsic defects associated with impurities. The analysis provide a framework for a physical understanding of thermal treatments on properties of sapphire crystals.

Z9.15
THE DIFFUSION MECHANISM OF OXYGEN IN ZRO2 GROWN ON ZR-2.5%NB. Hualong Li ,Michael G Glavicic, Jerzy A Szpunar, McGill Univ, Dept of Metallurgy Engineering, Montreal, CANADA.

Zr-2.5% Nb alloy Pressure tubes are used in the core of CANDU nuclear power reactors. During the operation, oxidation takes place. The oxide film, formed on the pressure tube surface is a good barrier against oxidation and hydrogen ingress. These processes have deleterious effect on the mechanical properties of the Zr-2.5% Nb alloy. It is known that the oxidation process in this alloy is controlled by inward diffusion of oxygen through the oxide grain boundaries. In this paper, the oxide grain boundary character distributions and grain boundary densities are calculated based on simulated oxide texture and microstructure. The results show that the oxide grain boundary character distribution and grain boundary density is affected by the alfa-Zr grain boundaries and beta-Zr impurities. These complex microstructural characteristics affect the diffusion process of oxygen through the oxide. The predicted oxidation kinetics has been compared to the experimental data and good agreement has been obtained.

Z9.16
ENHANCED OXIDE ION DIFFUSION IN THE INTERLANTHANIDE PEROVSKITE LaTmO₃. M. Deepa and U.V. Varadaraju, Materials Science Research Centre, Indian Institute of Technology, Madras, INDIA.

Substituted LaGaO₃ possesses oxide ion conductivity comparable to Calcia stabilized Zirconia [1]. Oxide ion vacancies that facilitate the oxide ion diffusion are created in the LaGaO₃ lattice by doping Sr at the La site and Mg at the Ga site. Mobility of the oxide ions in the isostructural LaAlO₃ is poor due to the smaller size of the unit cell which would hinder the diffusion of oxide ions. Also, since the covalency of Al³⁺, is higher, the oxide ions are held tightly in the lattice thereby decreasing the mobility of oxide ion. LaTmO₃ belongs to the well known class of Interlanthanide perovskite phases ABO₃. [2]. Since the ionic size of the Tm³⁺ (0. 88 Å) is much larger compared to that of Al³⁺ (0. 535 Å  in six fold oxygen coordination) and Ga³⁺ (0.62 Å) the diffusion of oxide ions in the interlanthanide lattice is expected to be more facile. With this view, single phase compounds in the system La_1-xSr_xTm_1-yMg_yO₃ (x=0-0.2, y=0.1 & 0.2) were synthesized. Ionic conductivity is significantly improved (by three orders of magnitude) with Sr doping suggesting that the diffusion pathway of the oxide ion motion is facilitated. This is found to correlate well with the results which showed highest oxygen diffision and catalytic activity for Sr doped LaBO₃ (3). However, further creation of oxide ion vacancies by doping Mg at the Tm site has not resulted in significant improvements on the conductivity. This could be attributed to the favouring of vacancy ordering, hindering the ionic mobility.
References:
1. Tatsumi Ishihara, Hideaki Matsuda and Yusaku Takita, J. Am. Chem. Soc. , 116 (1994) 3801.
2. H.K. Muller-Buschaum and CH. Teske, Z. Anorg. Allg. Chem. 369, (1969) 249.
3. J.B. Goodenough, H.Y.P. Hong and J.F. Kafalas,Mat. Res. Bull , 11 (1976) 203.

Z9.17
QUASIELASTIC NEUTRON SCATTERING BY SUPERIONIC PbF₂. Ion Padureanu, Aurel Radulescu, Sevastian Rapeanu, Annemarie Beldiman, Institute for Physics and Nuclear Engineering, Bucharest, ROMANIA; Zh.Kozlov, Joint Institute for Nuclear Research, Dubna, RUSSIA; V.A.Semenov, Institute for Physics and Nuclear Power, Obninsk, RUSSIA.

Some materials with the fluorite structure showing a pronounced specific heat anomaly at a temperature below their melting temperature have been the aim of an intense experimental and theoretical investigations. Though a large amount of experimental data exists however the conduction in fast-ion conductors is not fully elucidated. Therefore new experimental efforts are asked. In this paper new data obtained from the measurements taken on a high resolution TOF spectrometer set-up at IBR-2 Reactor of JINR Dubna are presented. The experimental investigations are performed on PbF₂ in normal (T=293 K) and in superionic state (T=823 K) by quasielastic neutron scattering. The choice of the initial energy of neutrons E₀=2.849 meV allowed us to obtain a resolution of $\Delta$E=0.148 meV for elastically scatered neutrons. An accurate analysis of the dynamical structure factor S(Q,$\omega$) in the (Q,$\omega$) space for constant Q as a function of h$\omega$ led to the conclusion that the quasielastic contribution of anion disorder in the superionic state of PbF₂ is observed for Q values higher than 2.1 A^-1. In the range of wave vector transfers 1.55$\leq$Q$\leq$2.05 A^-1 collective excitations like that detected in some simple liquids are observed as well defined symetrical peaks located at h$\omega\1\cong$(0.45- 0.55 meV). Thus it seems to be appropiate the ideea of the anion sublattice melting and its behaviour as a liquid. This could be the explanation of the fast ion conduction in the superionic conductors.



Z9.18
THE DC CONDUCTIVITY OF POTASSIUM TITANYL PHOSPHATE CRYSTAL ALONG ITS Z-AXIS. Qingcai Guan, Jiyang Wang, Weihong Cui, Jingqian Wei, Yaogang Liu, Xin Yin, Institute of Crystal Materials, Shandong University, P.R.CHINA.

The anomalous DC electrical conductivity of potassium titanyl phosphate (KTP) crystal along its z- axis is reported. The DC conductivity in the z- direction strongly depends on the measuring voltage while those in the x- and y- directions show no significant changes when the voltages vary from 0.1 to 1000v. The DC conductivity along z- axis is by 2 to 5 orders of magnitude higher than those along the other two principal axes. No Ohmic regime exists in the current-voltage relation in the z- direction even under very low applying voltages. Ionic polarization occurs under high DC stress which leads to the saturation of the current. An electrode reaction was also observed. The electrode material, Ag, was detected inside the crystal near the cathode by the micro-electron probe. The above phenomena are considered to be related to the easy movement of ions through the channel along the z- axis of KTP crystal.

Z9.19
CHEMICAL DIFFUSION IN $\beta$-Ag₂Te. Werner Sitte and Irmgard Rom, Institut für Physikalische und Theoretische Chemie, Technische Universität Graz, Graz, AUSTRIA.

Silver chalcogenides may be regarded as model materials for studying diffusion processes in mixed ionic-electronic conductors. We report on the results of composition dependent measurements of the chemical diffusion coefficient of the $\beta$-(low-temperature) modification of Ag₂Te within its homogeneity range between 110C and 140C. The chemical diffusion coefficients were obtained from galvanostatic polarization experiments using a symmetric solid state electrochemical cell with ionic electrodes and ionic probes. The composition of the sample was varied in situ with high stoichiometric resolution by means of the coulometric titration technique. As the electronic conductivity prevails the ionic one by orders of magnitude, the electronic transport number may be regarded as equal to one, which simplifies the calculation of the chemical diffusion coefficient from the voltage response of the polarization/depolarization experiments. The chemical diffusion coefficient $\~{D}$ of $\beta$-Ag$_{2+\delta}$Te shows a maximum around $\delta$=2.5x10^-4. The phase width ranges from approx. $\delta$=4x10^-4 to $\delta$=-15x10^-4 between 110C and 140C. This behaviour of $\~{D}$ is similar to the medium temperature $\alpha$ phases of Ag₂Te, although the absolute values of $\~{D}$ are about 3 orders of magnitude lower than those of $\alpha^\prime$-Ag₂Te at 160C or $\alpha$ Ag₂Te at 300C. The ionic conductivity of $\beta$-Ag₂Te at 140C (as obtained with the same experimental technique) is constant within the whole homogeneity range with values around $\sigma\1_i$=6x10^-4 Scm ¹. It can be summarized that regarding the chemical diffusion coefficient and the ionic conducitvity, $\beta$-Ag₂Te shows a similar behaviour like the structurally disordered alpha phases of the silver chalcogenides, although the chemical diffusion coefficients of the latter are in the order of magnitude usually observed in liquids.

Z9.20
SILVER DIFFUSION IN HTSC. P.P.Gorbik, I.V.Dubrovin, G.N.Kashin, V.M.Ogenko, Inst of Surface Chemistry, Kiev, UKRAINE.

Diffusion characteristics of HTSC are of great importance for understanding of their behaviour in engineering applications. In the present work the diffusion of silver in the YBCO and BSCO superconductors was studied. Silver thin films deposited on the surface of staked-plate HTSC single crystals by thermal evaporation were used as a diffusion source. The YBCO and BSCO crystal substrates have been prepared by crystallization of the melt of oxide precursors. The characteristics of diffusion process at the "silver film - HTSC single crystal" interface were determined by variations in the silver depth profiles after heat treatment at temperatures in the range from 100 to 500 centigrade degrees. The silver depth profiles were obtained by AES and argon ion sputtering. For the purpose of investigation of time dependencies of diffusion parameters the heat treatment was carried out for different time periods (from some minutes to some hours). In the YBCO samples the silver diffusion coefficient values are varied from 10^-16 to 10^-13 sq.cm/s in the temperature range investigated. In the BSCO samples the rate of silver diffusion is higher by the order of magnitude in the same temperature range and the diffusion coefficient values are varied from 10^-15 to 10^-12 sq.cm/s. The time dependencies of silver diffusion in the YBCO are shown that the initial diffusion rate determined for the 100 second period is almost tenfold of the average diffusion rate for more prolonged periods of heat treatment (up to 10⁴ s). In the BSCO system the reversed behaviour of diffusion rate vs. annealing time is observed. The possible influence of defect structure and surface composition of the HTSC single crystals and chemical interactions between silver and HTSCs are discussed. The analysis of sliver diffusion parameters in sintered HTSC materials was carried out. The experimental results were compared with the diffusion parameters for other chemical elements (In, Ni, Sn, Cd, Au, Bi, S, Se and others).

Z9.21
DIFFUSION AND PHASE FORMATION IN ME VS. MOSI₂ DIFFUSION COUPLES. Peter C. Tortorici, Hewlett-Packard Company, Microelectronics, Rohnert Park, CA; M.A. Dayananda, Purdue Univ., School of Materials Engineering, West Lafayette, IN.

Solid-solid diffusion couples assembled with disks of Me (Me = Mo, W, Re, Nb, and Ta) against both polycrystalline and single crystal MoSi₂ were annealed at selected temperatures between 1300&#186;-1700C. The diffusion structures that formed were characterized by microprobe analyses, SEM and optical microscopy, x-ray diffraction, and orientation imaging microscopy (OIM). The interdiffusion behavior of individual components in the binary and ternary silicides formed were evaluated with concepts of integrated and average effective interdiffusion coefficients. In the Me vs. MoSi₂ diffusion couples, both planar and non-planar morphologies were encountered in the development of multiple diffilsion layers, including a (Me,Mo)₅Si₃ layer. For the Mo₅Si₃ and W₅Si₃ layers formed in the ternary couples, OIM techniques revealed the development of 001 textures in the diffusion zone. In the (Me,Mo)₅Si₃ layers, Mo exhibited ``up-hill'' diffusion against its own concentration gradient in a direction opposite the flow of the other refractory element W, Re, Nb, or Ta. Such diffusion of Mo was characterized by negative interdiffusion coefficients. New observations of zero-flux planes without the formation of a relative extremum in concentration profiles are also reported for Mo in the Me vs. MoSi₂ couples.

Z9.22
GROWTH KINETICS OF $\epsilon$-FeSi and $\beta$-FeSi₂. M. Fanciulli and G. Weyer, Institute of Physics and Astronomy, University of Aarhus, Aarhus C, DENMARK; A. Zenkevich, Moskow Engineering Physics Institute, Moskow, RUSSIA.

Bulk stable and epitaxially stabilized iron-silicides have attracted a great deal of interest during the last decade due to intriguing fundamental problems as well as potential applications. Controversial results have been presented on the reactive Fe/Si(111)interface and its phase evolution upon thermal treatment and, despite the significant experimental effort, several questions remain unsolved. We report a ⁵⁷Fe conversion electron Mössbauer spectroscopy (CEMS) study of the kinetics of the phase transformations in the Fe/Si(111) system. Polycrystalline samples, where a 60 $\AA$ tracer layer of ⁵⁷Fe was grown at room temperature by pulsed laser deposition on Si(111) H-terminated surfaces, have been isothermally annealed in the temperature range of 250-500 C. The occurence and evolution of the different silicide phases has been monitored by ⁵⁷Fe CEMS. This work represents an extension of our previous investigation on the solid state reaction at the Fe/Si interface [1,2]. The particular thickness of the ⁵⁷Fe layer allowed a detailed investigation, from a microscopic point of view and with sub-monolayer sensitivity, of the early stages of formation of the different silicides. New information on the RT reactivity of the Fe/Si interface and on the nucleation and growth of the $\epsilon$-FeSi and $\beta$-FeSi₂ phases will be presented and discussed.

1. M. Fanciulli, C. Rosenblad, G. Weyer, H. von K{anel, N. Onda, V. Nevolin and A. Zenkevich, Mater. Res. Soc. Symp. Proc., Vol. 402, 319 (1995)
2. M. Fanciulli, S. Degroote, G. Weyer and G. Langouche, Surface Science 377-379, 529 (1997)

Z9.23
A STUDY OF DIFFUSION OF Li IN THE NZP NETWORK. G. Bhuvaneswari and U.V. Varadaraju, Materials Science Research Centre, Indian Institute of Technology, Madras, INDIA.

Phospahte compounds based on NaZr₂P₃O₁₂ (NZP phases AM₂P₃O₁₂) are interesting because of their potential applications including fast ion conduction.¹ The network structure consists of corner share PO₄ tetrahedra and MO₆ octahedra leading to interconnected interstitial space where the mobile sodium or other cations are located. There are two types of interstitial sites with distinct geometry referred to as type I and type II. Two diffusion pathways exists for the A cations through the interconnected interstitial sites. Earlier studies have shown that the type I - type II pathway has a low energy of activation and is most favoured route compared to the conduction path involving only the type II sites². It is the aim of the present study to explore the feasibility of Li⁺ diffusion through the type II pathway. This is best achieved by blocking the type I site by a bulky immobile cation like Pb2⁺ (6s² lone pair) and carrying out intercalation experiments. The presence of a transtition metal atom in the famework which can exhibit a variable valency paves the way to intercalate foreign elements like Li/Na into the lattice. A series of new phosphte compounds PbM³⁺ M⁴⁺ P₃O₁₂ (M³⁺ = Cr,Fe and In; M⁴⁺ = Ti, Zr, Sn and Hf) were syntheslzed. Intercalation of `Li' using n-BuLi in hexane as the reagent is performed at RT. One mole of lithium enters the network per formula unit of PbFeZr/HfP₃O₁₂ compounds. This amply demonstrates that the diffusion via type II pathway is indeed feasible. The observation has considerable bearing in designing new Li⁺ ion mixed conductors based on NZP structure. The experiments on the other phases are underway. Detailed results will be discussed.
References:
1. The Crystal structure of NaMe₂ ^IVP₃O₁₂; Me = Ge,Ti, Zr; L.O.Hagman and P.Kierkegaard, Acta Chemica Scand.,22,1822-1832, 1968.
2. Nasicon electrolytes: Part I the Na' diffusion path and its relation to structure Mat.Res.Bull., 20,1461-1471,1985.

Z9.24
DIFFUSION IN CERAMIC COATINGS OF GAS TURBINES BLADES. I. Kryukov, Al. Moshnikov, Art. Moshnikov, A. Rybnikov, The Polzunov Central Boiler and Turbine Institute, St. Petersburg, RUSSIA.

Multilayer coating with an external ceramic layer find the increasing application in power mechanical engineering, thus as a ceramic layer ZrO₂ stabilized Y₂O₃ is perspective. It is connected that ZrO₂ has low thermal conductivity and it can effective carry out functions of a thermal barrier. Besides on temperature factor of linear expansion this material is close Al₂O₃, that provides satisfactory adhesion at occurrence Al₂O₃ because of diffusion of oxygen through a ceramic layer. The results of development of theoretical representations about diffusion of oxygen in conditions of a gradient of concentration and gradient of temperatures through partially binding border are given, the experiments on study of formation of intermediate layers Al₂O₃ by a method EPMA are carried out.  The influence of the cavity sizes in a ceramic layer on efficiency of absorption of compound components of process sulphide-oxide corrosion is marked.

Z9.25
A CONTROLLED SOLID STATE DIFFUSION PROCESS TO FORM CERMET ANODES FOR SOLID OXIDE FUEL CELLS. Eric Z. Tang, Douglas G. Ivey, Thomas H. Etsell, University of Alberta, Dept of Chemical and Materials Engineering, Edmonton, AB, CANADA.

The interfacing of thin film vapor deposition technologies and solid state electrochemistry has led to the recent development of polarized electrochemical vapor deposition (PEVD). In this study, PEVD has been applied to deposit a thin layer of yttrium stabilized zirconia (YSZ) over a porous metallic (Pt, Co or Ni) electrode as a cermet anode of a solid oxide fuel cell (SOFC). During PEVD, oxygen ions are transported through the solid electrolyte of an SOFC under an electrical potential gradient provided by an external dc source. At the metallic electrode (anode) surface, oxygen ions react electrochemically with zirconium chloride and yttrium chloride vapors to deposit YSZ. The growth of YSZ follows the mechanisms described in Wagner's tarnishing theory, i.e., the minority charged species in YSZ controls its growth rate. Modification has been made to the initial growth of YSZ at both electronically and ionically shorted paths along the metal electrode and solid electryte surfaces respectively. The thin layer of deposited YSZ provides an ionic conducting path to the entire surface of the metal electrode. Consequently, the electrochemically active sites at the anode are markedly increased. Both ionic and electronic conducting paths are continuous at the anode and the ohmic loss is smaller than for state-of-the-art anodes formed by slurry coating of both YSZ and metallic particles. Vapor loss and sintering of the metallic electrode are minimized by covering with YSZ,
which increases the long-term stability of the anode.

SESSION Z10: DIFFUSION AND IONIC CONDUCTIVITY IN IONIC MATERIALS
Chair: C.R.A. Catlow
Thursday Morning, April 16, 1998
Golden Gate C1

8:30 AM *Z10.1
DETERMINATION OF IONIC DIFFUSION MECHANISMS IN SOLIDS. John Corish, Department of Chemistry,Trinity College, University of Dublin, Dublin, IRELAND.

The classical experimental determination of ionic diffusion mechanisms in crystalline solids of known crystal structure is through the evaluation of the correlation factor for the migration process. This requires unambiguous measurements over a range of temperature of both the diffusion coefficient and the contribution by the species to the overall ionic conductivity. Typically very careful data analyses are necessary to separate the contributions of different species and of the same species moving via different mechanisms. Such determinations, made on classical ionic conductors such as the alkali and silver halides, have greatly enhanced our knowledge of ionic defect formation, interaction and transport processes. A full understanding of the atomistic mechanisms through which ions migrate remains crucial for the design of new materials in which transport processes are optimised, for example in the current development of the oxygen ion conductors used in SOFC for energy conversion. The advent of reliable computational atomistic simulation techniques had an immediate effect on the determination of ionic diffusion mechanisms. These techniques can be applied in materials in which measurements are either difficult or which are so complex that interpretations of experimental data are uncertain. Static lattice calculations require the specification of migration pathways whereas mechanisms emerge naturally in cases in which the techniques of molecular dynamics can be used. In suitable cases not only can the nature of the mechanism be determined but the calculations also reveal details of the pathway followed by the migrating ion. Again such information may be vital when new materials are designed to provide efficient ion migration for specific ions. The current state of our understanding of ionic diffusion mechanisms, the experimental and computational techniques available for their determination and the applications in which such knowledge is useful will be discussed.

9:00 AM Z10.2
Abstract Withdrawn

9:15 AM *Z10.3
PROTON AND OXYGEN ION TRANSPORT IN MIXED METAL OXIDES. M. Saiful Islam, University of Surrey, Dept of Chemistry, Guildford, UNITED KINGDOM.

Computational techniques are now well established tools for probing the structural and transport properties of solid state ionic materials. This presentation will highlight recent developments of such methods, with a strong emphasis on their use in the elucidation of defect structures and diffusion pathways at the microscopic level. Current work will be illustrated by focusing on two ceramic oxide systems of much interest: (i) oxygen ion and proton migration in LaMO3 (M = Mn, Co, Ga) perovskite-type materials which find use in solid oxide fuel cells and oxygen separation membranes, (ii) proton and lithium insertion in the spinel-structured MnO2 - LiMn2O4 system which has potential battery applications. We have used atomistic simulation, molecular dynamics (MD) and ab initio techniques to provide fresh insight as to their defect and ion diffusion behaviour. The examination of a range of dopant substitutions and defect clusters will also be discussed.

9:45 AM Z10.4
IONIC CONDUCTIVITY STUDY ON THE NEW OXIDE ION CONDUCTING PEROVSKITE LaGaO₃. G.V.M. Kiruthika and U.V. Varadaraju, Materials Science Research Centre, Indian Institute of Technology, INDIA.

Oxide ion conductivity higher than that of hitherto known YSZ has been discovered in the doped perovskite LaGaO₃(1). Aliovalent substitution of Sr at the La site and Mg at the Ga site with the composition, La_0.9Sr_0.1Ga_0.8Mg_0.2O$_{3-\delta}$ (LSGM) has given a remarkable conductivity of 0.11 $\Omega\1^{-1}$ cm^-1 at 800&#186;C(2). It is generally believed that oxygen diffusion is based on rapid transport of vacancy, which migrates by a conventional hopping mechanism, although this has not been confirmed by experirnents. It has been recently confirmed by M.Cherry et al., (3) that, ion diffusion in perovskite type of oxides (LaBO₃) arises from the mobility of oxide ion vacancies. An important result of this search of potential energy is that, they find a small deviation from the direct path of their vacancy migration. Calculations have revealed that the migrating ion, instead of taking a linear path along the edge of the BO₆ octahedra, takes a curved route with the saddle point energy away from the neighboring B site cation, resulting in a signiflcantly lower energy barrier to oxygen migration. A lower migration energy is favoured with smaller A site cations and larger B site cations (3). Ionic size effect was found to be important for dopant substitution. The activation energy for LSGM was found to be higher than that of YSZ (2). A lower activation may be achieved by providing a strain free pathway for oxide ion motion. Attempts have been rnade to substitute (10-20%) Y (ionic radious - 0.90&#186;A) at the Ga (0.62A) site. This may be thought of to increase the size of the unit cell and hence provide an optmal bottle-neck size for oxide ion migration. La_0.9Sr_0.1(Y_x7Ga_1-x)_0.8Mg_0.2O₃ (x=0,0.l & 0.2) were synthesized by high temperature solid state method and the phase purity was established frorn X-ray diffraction. Yttrium substitution at the Ga site indeed results in the decrease of the activation energy with a marginal change in conductivity. Results of the study will be discussed.
References:
1. T. Ishihara, H. Maduda and Y. Takita, J. Am. Ceram. Soc. , 116 (1994) 3801.
2. M. Feng and J.B. Goodenough, Eur. J. Soltd State Inorg. Chem. , T31 (1994) 663.
3. M. Cherry, M.S. Islam and C.R.A. Catlow, J. Solid State Che. , 115 (1995) 125.

10:30 AM *Z10.5
INTERPLAY BETWEEN ANION ROTATION AND CATION TRANSPORT IN THE PLASTIC HIGH-TEMPERATURE PHASE OF SODIUM ORTHO PHOSPHATE. K. Funke, D. Wilmer, R.D. Banhatti, M. Witschas, H. Eckert, Müenster University, Institute of Physical Chemistry, Müenster, GERMANY; R.E. Lechner, J. Fitter, Hahn-Meitner Institute, Berlin, GERMANY; M. Jansen, G. Korus, Bonn University, Institute of Inorganic Chemistry, Bonn, Germany; M. Parrinello, Max-Planck-Institut für Festkoerperforschung, Stuttgart, GERMANY.

The high-temperature phase of sodium ortho phosphate, $\rm Na_3PO_4$, belongs to the class of ion conducting plastic crystals, i.e., it is characterized by a dynamic rotational disorder of its polyatomic anions and, at the same time, by a considerable translational mobility of its cations. During the past decade, the possibility, nature, and importance of a dynamic interplay between the two kinds of motion have been a subject of continued controversy. Proponents of a strong interplay coined the expression ``paddle-wheel mechanism''. In our present contribution we report, for the first time, on the results of dynamic experiments probing the elementary steps of anionic and cationic motion individually. The techniques utilized in this study are coherent quasielastic neutron scattering and high-frequency conductivity spectroscopy, respectively. The data are complemented by solid-state NMR and Car-Parrinello-type molecular-dynamics simulations. Our results provide a detailed description of the anionic and cationic modes of motion. They strongly support the view that moving sodium ions are peripherally attached to rotationally mobile phosphate ions.

11:00 AM Z10.6
MOLECULAR DYNAMICS STUDIES OF Cd_1-xPb_xF₂ SYSTEMS. T.T. Netshisaulu, P.E. Ngoepe, Department of Physics and Materials Modelling Centre, University of the North, Sovenga, SOUTH AFRICA; A.V. Chadwick, School of Physical Sciences, University of Kent, Canterbury, UNITED KINGDOM.

PbF₂ and CdF₂ undergo transition to the fast-ion phase above 700 and 1100K respectively. Their mixture yields pronounced reduction of these transition temperatures (T_cs), and considerable enhancement of ionic conductivity below T_c. Molecular dynamics study is being used to investigate diffusion of fluorine ions in Cd_1-xPb_xF₂ (for x = 0-1) as a function of concentration and temperature. Diffusion coefficients, predominant ion migration mechanisms and radial distribution functions will be presented. EXAFS studies have been used to probe local environments around Cd and Pb cations in this system. The measured radial distribution functions are consistent with findings from computer simulation studies.

11:15 AM Z10.7
HOST CATION AND DOPANT CATION TRACER DIFFUSION IN SINGLE CRYSTALLINE CUBIC STABILIZED ZIRCONIA. Martin Kilo, Gunter Borchardt, TU Clausthal, Institut für Allgemeine Metallurgie, Clausthal-Zellerfeld, GERMANY; Sylvain Weber, Stanislas Scherrer, Ecole des Mines de Nancy, Laboratoire de Metallurgie Physique, Science des Materiaux, Nancy, FRANCE.

Because of its high oxygen conductivity, stabilized cubic zirconia is of interest as a solid electrolyte for high temperature electrochemical applications. Consequently, many investigations on oxygen diffusion or ionic conductivity were carried out. In contrast, very little is known in this material about cation diffusion, which governs the long-term electrical and mechanical performance (ageing and creep). Therefore, zirconium tracer diffusivities were measured in yttria stabilized single crystalline cubic ZrO₂ as a function of stabilizer concentration and temperature. As a tracer, the stable isotope ⁹⁶Zr was used. The diffusion profiles were analysed with SIMS. In the temperature interval 1200 &#186;C < T < 1500 &#186;C the activation energy for zirconium tracer diffusion varies between 4.4 and 5.1 eV without significant dependence on the yttria content (10 - 32 mol %). For a given temperature the diffusivity decreases with yttria content. This can be explained by migration via a complex defect (Vo^2-,V$_{Zr}\1^{4+}$)²⁺ containing vacancies on both sublattices. At lower temperatures, evidence is found for the existence of another diffusion mechanism with lower activation energy. Further, results on calcium and zirconium tracer diffusion in calcia stabilized cubic zirconia are presented.

11:30 AM Z10.8
ENHANCEMENT OF IONIC DIFFUSION BY MICROWAVE-FIELD-INDUCED PONDEROMOTIVE FORCES AT PHYSICAL INTERFACES. John H. Booske^(a,c), Reid F. Cooper^(b,c), Samuel A. Freeman^(c), Kimberley R. Binger^(a), Hang Guo^(a), and John Hickland^(a),^(a)Dept. of Electrical and Computer Engineering,^(b)Dept. of Materials Science and Engineering, and^(c)Materials Science Program, University of Wisconsin, Madison, WI.

Numerous observations have been reported in the literature of enhanced mass transport and solid-state reaction rates during microwave heating or processing of a variety of ceramic, glass, and solid state materials. This talk will describe a recent body of research that has identified, isolated, and characterized a previously unknown driving force for ionic mass transport. The driving force-termed a ìponderomotiveî (mass-moving) force-results from the application of intense, high-frequency electric fields near structural interfaces (e.g., free surfaces, grain boundaries). Experiments, theory, and numerical simulations all demonstrate that this driving force can influence solid state reaction kinetics by enhancing mass transport rates. Most of the previously reported observations of nonthermal solid state reaction kinetics during microwave heating are consistent with the characteristics of the microwave ponderomotive force.

11:45 AM Z10.9
MOLECULAR DIFFUSION PROCESSES IN CRYSTALLINE MICROPOROUS MATERIALS. C.R.A. Catlow, The Royal Institution of Great Britain, London, UNITED KINGDOM; G. Sastre, A. Corma, Instituto de Tecnologia Quimica, Universidad Politecnica de Valencia, Valencia, SPAIN; E. Hernandez, Fisica Teorica, Universidad de Valladolid, Valladolid, SPAIN.

Crystalline microporous materials (including aluminosilicate, zeolitic and aluminophosphate systems) are widely used in both catalysis and gas separation technologies[1]. Of crucial importance in both these applications is the rate of molecular diffusion processes within the pores of these solids. Despite considerable progress in the experimental identification of diffusion coefficients of sorbed species, there remain several difficulties in obtaining unambiguous data. For this reason, computer simulation methods are playing an increasingly important role in this field[2]. In this paper we discuss two important systems where we have probed diffusion rates and mechanisms using molecular dynamics simulations: the first concerns the diffusion of n-alkanes through the widely studied high silica material silicalite; the second, the diffusion of aromatic hydrocarbons through the recently synthesised material CIT-1. Our simulation allows us to determine diffusion coefficients and the effect on them of temperature and loading. Mechanistic aspects are also explored and our results highlight the crucial role of the channel dimensions and connectivity in controlling diffusion rates.
[1] Thomas, J.M., (1992), ScientificAmerican , 4, 112.
[2] Modelling of structure and reactivity in zeolites, Ed. Catlow, C.R.A., (Academic Press Limited, London) 1992.