Effect of Covalency on Phonon Dispersion Relations in NaCl Type Alkali Halide Crystals

Abstract

A lattice dynamical model is constructed for the NaCl type alkali halide crystals by incorporating the covalency effects in the framework of Hardy’s polarization dipole model. Three effects of covalency are taken into account: (1) the reduction of the effective charge of the long range Coulomb interaction between ions from the formal charge of ion, (2) the attractive force due to covalency, and (3) the field induced charge flow. The model is found to provide a closer description of the dispersion curves than the deformation dipole model that is obtained by incorporating the mechanical polarization effects in the framework of the polarization dipole model.