Comparisons between shell and deformation dipole models for the thermal conductivity of alkali halides and its volume dependence

Accurate dispersion relations have been used for the first time in a calculation of the volume dependence of the thermal conductivity in 16 alkali halides with the NaCl structure. Phonon dispersion relations have been calculated throughout the whole first Brillouin zone for different values of the lattice constant. These have been used to solve the Boltzmann equation by a variational method and to obtain the thermal conductivity for different lattice constants. The author has used both a deformation dipole model and a shell model and compares the results obtained with them. He discusses various contributions to the volume dependence of the thermal conductivity and finds good agreement with room-temperature experiments. The differences in this volume dependence among the alkali halides are mainly caused by changes in the three-phonon scattering rate with volume whereas changes in the anharmonic potential give approximately the same contribution to all alkali halides studied.