The crystal dynamics of lead sulphide

Abstract

The frequencies of normal modes propagating in the high symmetry directions, [001], [110] and [111], of lead sulphide have been measured by inelastic neutron scattering techniques. A marked decrease in the value of the longitudinal optic mode frequency has been observed at small wavevectors. A similar effect has been reported in lead telluride, and is attributable to the screening of the macroscopic electric field by the free carriers in the crystal. The longitudinal optic mode frequencies have been estimated for the particular case of zero carrier concentration. The corrected frequencies have been used to deduce the parameters of rigid ion models and shell models for the interatomic forces of lead sulphide. The quality of the fit for both types of models is less satisfactory than for the equivalent models used for the alkali halides, but is comparable with those obtained for lead telluride. The best model gives a good fit to the frequencies, and has been used to calculate the frequency distribution and specific heat of lead sulphide. There is a marked similarity between some of the models of lead sulphide and lead telluride. One of these models has been used to calculate dispersion curves for lead selenide, and fair agreement has been obtained with the available experimental data.