Thermodynamics and Defect Chemistry of Some Oxide Solid Solutions Part III. Defect Equilibria and the Formalism of Pair Interactions

Abstract

AbstractThis paper is concerned with intrinsic disorder, deviations from stoichiometry and substitutional disorder in ionic solids and solid solutions. The assumptions are made that the distribution of ions over the sites of any sublattice is random and that the free enthalpy of the solids is the sum of (a) the “standard” free enthalpy of the ions and vacancies, (b) pair interactions between the ions and vacancies, and (c) the configurational entropy term. Proof is given that in such solids and solid solutions the standard free enthalpy of each defect equilibrium is a linear function of all compositional and disorder parameters, which is due to the pair interactions.A comparison is made with some experimental data given in the literature. In particular, solid solutions of two isostructural oxides are considered, which show substitutional disorder of the type of exchange of unlike cations between unlike sites. Numerous literature data about cation distribution over different sublattices in spinels, garnets, barium ferrites and orthopyroxenes are compiled. They agreed with the theoretical equations and the values of the apparent pair‐interaction energies were considerable. The applicability of the assumptions and the nature of these pair interactions are discussed.