CALCULATION OF ABSOLUTE DIFFUSION RATES IN OXIDES

The authors have calculated the absolute rate of diffusion for Mg2+ and Fe3+ ions in MgO using atomistic modelling. The calculations use a shell model, incorporating tested interatomic potentials, and exploit recent advances in computer codes. The agreement is extremely good where experimental data are available for comparison. For Mg2+ in MgO at 1400 degrees C they predict a pre-exponential factor of 32.9 THz using simple Vineyard theory (experiment 18+or-7 THz after correction for the important volume dependence of the activation energy) and an activation energy of 2.26 eV (experiment 2.3+or-0.2 eV). Close inspection of the energy changes for displacements from the saddle point normal to the jump path shows that within kT of the saddle point energy there are significant departures from the harmonic dependence required for validity of Vineyard theory. Corrections by both analytical methods and numerical integration improve agreement with experiment, predicting 23-25 THz overall. For Fe3+ much slower diffusion is predicted even though the jump path bifurcates to give two saddle points. The authors do not predict the rapid Fe3+ motion reported in aggregation experiments and conclude that other mechanisms are involved. They have also used the dynamical theory of Rice and Slater which gives similar, but by no means identical, predictions for the diffusion rates

CATION DIFFUSION IN ALKALINE-EARTH OXIDES

Absolute jump rates for cation-vacancy interchanges in MgO, CaO, SrO and BaO are calculated from a set of model inter-ionic potentials. Internal energies and vibrational entropies over a wide range of temperatures (i.e. at expansions which within the models correspond to these temperatures in the quasi-harmonic approximation) are evaluated and, from these, migration enthalpies and pre-exponential frequency factors are deduced. Correlations between these two diffusion parameters for the family of oxides are investigated

Vibrational pocket modes: predictions by the embedded crystallite method and their experimental observation

Simulation studies based on the embedded crystallite method are used to predict, with no free parameters, complex dynamical behavior for a simple alkali halide defect system, Na+ in KI. Far infrared spectroscopic measurements, including uniaxial stress, confirm the predicted vibrational properties, indicating that this methodology can readily be used for complex and extended defects in ionic crystals

Shell-model calculation of defect energies in alkali halides employing crystal-independent interionic potential parameters

Shell model calculation of defect energies in alkali halides have been carried out using the ion-dependent, crystal-independent potential parameters of Sangster and Atwood (1978). Results indicate that appreciable differences exist between barrier heights for migration of cations and anions. While barrier heights for cations are generally lower than for anions in alkali halides of NaCl structure, the opposite is true in alkali halides of CsCl structure