Lattice Dynamics of NaCl, KCl, RbCl, and RbF

Abstract

In a series of earlier papers, experimental results on phonon dispersion relations at 80 °K in NaCl, KCl, RbCl, and RbF have been reported. We now present calculations on these halides using the extended-shell-model approach with both ions polarizable and including next-nearest-neighbor interactions. The parameters obtained in a least-squares fit to the experimental points in the symmetry directions have been used to calculate the phonon frequencies in 512000 equally spaced points in an octant of the Brillouin zone, whereby, after sorting these into intervals of width $\Delta \omega =2\mathrm{}\times {10}^{11}$ rad ${\sec }^{-1\mathrm{}}$, the frequency spectrum was obtained. From these spectra the variation of the Debye temperature with temperature was also calculated. The agreement with results from specific-heat measurements for NaCl and KCl is quite satisfactory at low temperatures.