Stability of Crystals of Rare-Gas Atoms and Alkali Halides in Terms of Three-Body Interactions. II. Alkali-Halide Crystals

Abstract

By extending the theory developed in a previous publication for the stability of rare-gas crystals, it is shown that the stability of alkali-halide crystals can be explained in terms of three-body exchange interactions between the ions. As in the case of rare-gas crystals, the analysis is based on a first- and second-order perturbation calculation with a Gaussian effective-electron model. The different size of anion and cation of each solid is taken into account. The effect on stability of double-exchange contributions to the three-body energy (negligible for rare-gas crystals) is analyzed in detail. It is shown that the theory accounts for all observed regularities on a quantitative basis. In particular, cesium chloride, bromide, and iodide are found to be stable in the cesium chloride modification; furthermore, calculated and observed values for the pressure of transition from the sodium chloride to the cesium chloride configuration are in good agreement.