Tracer diffusion via bound vacancy pairs in mixed ionic crystals

In this paper we adapt Manning's diffusion kinetics formalism in the random alloy to produce a two-tiered diffusion kinetics formalism in mixed ionic crystals with diffusion being mediated by bound vacancy pairs. Both NaCl an d CsCl structures are dealt with. Building on our previous formalism to des cribe diffusion in the binary ionic crystal we describe diffusion correlati on effects in a situation where there can be two types of monovalent cation and two types of monovalent anion competing for the bound vacancy pair. We assume that the two types of cation are randomly distributed, similarly fo r the two types of anion. Tracer correlation factors are expressed in close d form in terms of the sublattice atom-vacancy pair exchange frequencies, t he relative compositions of atomic species and the geometric correlation fa ctor. The results are in good agreement with Monte Carlo simulation results also determined in the present study. For the ternary mixed system we show how ratios of tracer diffusion coefficients can be analysed to give tracer correlation factors explicitly. The percolation characteristics of the ter nary system are also commented on.