Lattice Dynamics and Specific-Heat Data for Rocksalt-Structure Alkali Halides

Vibrational distribution functions are derived for a number of rocksalt-structure alkali halides using a more refined treatment of the interionic forces than that provided by regarding them as rigid point charges. The dipole moment at any given ion site is calculated taking into account the contribution from the deformation of the electron distribution resulting from both polarization and overlap repulsion between nearest neighbors. In this way the dipole-dipole part of the Coulomb interaction is treated self-consistently. Both room temperature and 0°K input parameters are used, and the derived specific-heat data are compared with experimental results. The over-all agreement with experiment is significantly better than that obtained by treating the ions as rigid point charges. Sets of phonon dispersion curves are also given. For NaI they are in much better agreement with those determined experimentally by inelastic neutron scattering than are the rigid ion curves. There appears to be close agreement with the results of the shell-model calculations