It was recalled that, in strongly ionic insulating materials, the Nernst-Planck equation related the interdiffusion coefficient of the cations (having the same charge) to the corresponding tracer diffusivities and a thermodynamic factor. The Nernst-Planck equation for ionic ternary (quasi-binary) and ionic quaternary (quasi-ternary) systems were explored here by using the diffusion kinetics formalisms of Darken (1948), Manning (1971) and Moleko et al. (1989)]. It was shown that, for the binary system, the Darken and Manning formalisms both gave the usual form of the Nernst-Planck equation. However, the almost-exact Moleko, Allnatt and Allnatt formalism (for randomly mixed systems) provided an additional correction factor which was analogous to the vacancy-wind factor in the well-known Darken-Manning equation for binary alloy systems. Nernst-Planck-type equations were also derived for strongly ionic insulating ternary systems, and were found to behave in a similar manner to that for the binary case.

Interdiffusion in Strongly Ionic Insulating Compounds - the Nernst-Planck Equation. I.V.Belova, A.R.Allnatt, G.E.Murch: Philosophical Magazine, 2007, 87[27], 4169-80