The volume of formation of defects in ionic crystals

Abstract

Abstract Calculations on the volume of formation of Schottky pairs in alkali halides and Frenkel pairs in alkaline earth halides are reported. They are based on the relation between the volume of formation and the derivative with respect to crystal volume of the energy of formation. In contrast to earlier calculations, which used a different method, the present ones give a positive relaxation volume for Schottky pairs in alkali halides, as required by experiment, the magnitude of this volume being also qualitatively correct. The formation volume of Frenkel pairs in the salts CaF2, SrF2 and BaF2 is calculated to be about a quarter of a molar volume, in agreement with experiment. It is pointed out that the formula used for the formation volume can be helpful in clarifying the relation between measured and calculated formation energies.