Physics
Scientific paper
Apr 1981
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1981pepi...25...38k&link_type=abstract
Physics of the Earth and Planetary Interiors, Volume 25, Issue 1, p. 38-51.
Physics
17
Scientific paper
A continuum model of point defects in ionic solids is developed and applied to the estimation of the pressure dependence of diffusion (in alkali halides). Activation enthalpies are expressed using macroscopic physical constants (lattice energy, elastic constants, dielectric constants, etc.) by introducing empirical parameters related to the microscopic nature of defects. Systematic relations are found between these parameters and other physical properties (ionic radius, volume fraction of anions in crystals, etc.). The nature of parameters for anions is shown to be significantly different from that for cations, probably due to the difference in ``softness'' between anions and cations. Such relations make it possible to estimate activation energy and volume of diffusion from the lattice energy, ionic radius, elastic constants, dielectric constants and their pressure derivatives. The calculated activation volumes agree well with experimental data. The effect of phase transition is also examined by comparing NaCl and CsCl structures. It is suggested that by phase transition from a less dense (NaCl) structure to a denser one (CsCl), the anion diffusion constant may not decrease as much as expected from the associated change in other physical properties. The validity of the models of van Liempt and of Keyes is examined. It is shown that van Liempt's model works well except for the migration process (especially anions), and that Keyes' model does not work so well for all processes.
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