INFLUENCE OF SHORT-RANGE 3-ION INTERACTIONS ON POLYMORPHISM AND ELASTIC CONSTANTS OF SIMPLE SOLIDS .I. ALKALI HALIDE CRYSTALS

A theoretical analysis is made of crystal stability and of low-temperature deviations from the Cauchy relations between elastic constants of ionic crystals with AX composition and with cubic or hexagonal structure. The Born-Mayer model is used as a basis, extended to take into account short-range three-ion interactions of exchange type. These interactions are evaluated in first and second orders of perturbation theory, adopting a Gaussian effective-electron approximation. In the stability analysis, four different crystal structures (B1, B2, B3, and B4,) are compared for each compound. In this paper, the results are reported for alkali halide crystals. Good agreement with experimental results is found. A following paper will deal with solid ionic compounds whose cations possess a complete outer d shell of electrons. © 1969 The American Physical Society.