Thermodynamic Properties of the Alkali-Halide Crystals

Abstract

The equilibrium properties of fourteen alkali halides in the NaCl lattice structure are calculated by using an expression for the pair potential given by Tosi and Fumi. The calculations are based on the quasiharmonic lattice-dynamic method and the unsmeared Lennard-Jones-Devonshire cell model. The latter provides numerical estimates of anharmonic contributions which are neglected in the former. Comparison of the calculations with the available experimental data as well as the Monte Carlo data of Woodcock and Singer shows reasonable agreement for the cohesive energy, the pressure, the specific heats, the coefficients of thermal expansion, and the Grüneisen $\gamma$'s, but relatively poor agreement in the case of the elastic constants. Our results indicate that the interionic potential for the alkali halides has a stiffer repulsive core and a stronger attractive tail than the expression given by Tosi and Fumi, and that the anharmonic corrections are generally small and should be described accurately by the two lowest terms in a suitable perturbation series. Further discussion is given on the accuracy of using an existing approximate theory for the elastic constants and on the self-consistency of making the quantum and anharmonic corrections introduced here.