Phonon Frequencies in NaBr

Abstract

The frequencies of phonons in NaBr at 295\ifmmode^\circ\else\textdegree\fi{}K propagating along the [001], [110], and [111] directions have been obtained from measurements of slow-neutron scattering made with a time-of-flight spectrometer and with a triple-axis spectrometer. The results have been fitted by least-squares techniques to both rigid-shell and deformable-shell models. The most satisfactory fit is obtained using a rigid-shell model with six disposable parameters and no second-neighbor force constants. Frequencies given by a variety of models with parameters determined by macroscopic constants are compared with the measured frequencies, and the effect of using zero-sound elastic constants is examined. The agreement is comparable to that found in other polarizable alkali halides. Finally, the frequency distribution at room temperature is obtained, and the temperature dependence of the specific heat and of the reduced moments are calculated with allowance for temperature-dependent quasiharmonic frequencies.