Correlated hopping conductivity in a general two sublattice structure

We show for a general ordered two sublattice model with inequivalent A and B sites that the classical hopping conductivity is anomalously low at stoichiometry (total number of mobile ions equal to total number of A sites). The activation energy increases from its value V/sub A/ for a mobile ion concentration well off stoichiometry to V/sub A/+1/2(V/sub A/-V/sub B/) at stoichiometry, where V/sub A/ and V/sub B/ (V/sub A/>V/sub B/) are depths of the potential wells at A sites and B sites. This is a generalization to arbitrary dimensions and coordination number of similar results obtained by Sato and Kikuchi for the two dimensional β-alumina structure and by the author for a linear chain. As an example, the relevance of recent experiments on the alkali zirconium phosphates is discussed