The Crystal Dynamics of Lead Telluride

The frequencies of the normal modes of vibration of lead telluride propagating in certain symmetric directions have been determined by inelastic neutron scattering techniques. The results have been used to deduce the parameters of rigid ion models and of shell models for the interatomic forces. The latter models take account of the polarizability of the ions. Neither type of model is very successful, but the best shell model does provide a good fit to the results and has been used to calculate the frequency distribution and specific heat. Calculations of the thermal expansion, and of the lifetime of the transverse optic modes, show that anharmonic interactions in lead telluride are considerably more complex than in the simple alkali halides. A detailed account of the effect of doping on the lattice vibrations is given, in which it is shown that large effects are limited to the long wavelength regions of the dispersion curves. Detailed calculations are presented for the dispersion of the optical branches as a function of the dopant concentration. The importance of these effects for different kinds of experimental studies is briefly discussed.