A calculation of defect Gibbs energies for silver chloride and silver bromide

Abstract

Computer simulation techniques have been used to calculate the energy and entropy changes, at constant volume, for the formation of a Frenkel defect, for the association of a cation vacancy with a cation impurity, and for the frequency of the exchange of a cation impurity with a nearest-neighbour cation vacancy in both silver chloride and silver bromide. The results are used to compute the corresponding Helmholtz energy changes at constant volume, which may (to a good approximation) be identified with the corresponding Gibbs energies at constant pressure, as derived from experimental measurements of ionic transport. The impurities considered were Cd²⁺, Na⁺ and Cs⁺ in AgCl and Cd²⁺ Na⁺, Rb⁺ and Cs⁺ in AgBr. The results were also used to calculate impurity-diffusion coefficients for these systems. While the results of these calculations possess many encouraging features, it is clear that the difficult problems involved in modelling the silver halides have not yet been solved entirely satisfactorily.