Influence of Short-Range Three-Ion Interactions on Polymorphism and Elastic Constants of Simple Solids. II. Compounds with Closedd-Shell Cations

The analysis given in the previous paper of polymorphism and low-temperature deviations from the Cauchy relation $c_{13}=c_{44}$ between the elastic constants of alkali halide solids is extended to ionic crystals of composition $\mathrm{AX}$ whose cations possess closed outer $d$ shells of electrons, i.e., monovalent copper and silver, and divalent zinc and cadmium. The Born-Mayer model is again taken as a basis, supplemented by three-ion, exchange, non-Coulombic interactions in first and second orders of perturbation theory. It is found that attractive, exchange, cation-cation, and cation-anion pair interactions together with the three-ion forces explain polymorphism and the deviations from the Cauchy relations in cubic and hexagonal structures.