ON THERMODYNAMIC PROPERTIES OF SOLID ELECTROLYTE RBAG4I5

The non-random distribution of Ag+ ions on three crystallographically nonequivalent sets of tetrahedral sites in RbAg4I5 is a consequence of both site energy differences and mutual repulsion of silver ions on nearest neighbor sites. A method similar to the quasichemical approximation, used in the theory of order-disorder transformations, has been applied to obtain the eonfigurational entropy and excess specific heat associated with the disorder of the silver ions. Site energy differences of ∼ 0.027 and ∼ 0.042 eV between the different types of silver sites and a mutual repulsion energy between silver ions on nearest neighbor sites of ∼ 0.035 eV accounts well for the experimentally observed quantities. The configurational entropy is an appreciable fraction (∼ 10 per cent at 300°K) of the total entropy content. © 1969.