Interionic potentials and force constant models for rocksalt structure crystals—II

A lattice dynamical model for rocksalt structure crystals is presented in which the interionic force constants are taken to be central and the departure from the Cauchy relations is accounted for by isotropic and anisotropic deformations of the anions. The model is applied to NaBr, NaI and KBr for which C12 > C44 and it is appropriate to consider only the anisotropic deformations. The model parameters are determined entirely from macroscopic data. By assuming an exponential form for the repulsive part of the interionic potential the cohesive energy of the crystals may be determined from the interionic force constants. The cohesive energies are in surprisingly good agreement with experiment although the potential parameters differ considerably from those of other workers. While the dispersion curves obtained from the model are in markedly worse agreement with experiment than those from the standard breathing shell model it is shown that the potentials(in addition to giving good cohesive energies) result in a reasonably consistent dynamical model.