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W-Ojn-sT PPPL-2123 ' KOTlCt PORTIONS OF THIS REPORT ARE ILLEGIBLE, it fiaTblSirSproftuced trom 5he best available copy Is permit tho broadest passible avail ability.

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 (Bl, 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. 1150The analysis given in the previous paper of polymorphism and low-temperature deviations from the Cauchy relation cn = cu between the elastic constants of alkali halide solids is extended to ionic crystals of composition 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 threeion, 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.

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