LATTICE DYNAMICS OF TIN TELLURIDE

The frequencies of normal modes of vibration propagating in high symmetry directions in tin telluride at 100°K have been measured by means of inelastic neutron scattering techniques. The shape of the longitudinal optic branches is dominated by the screening effects of the free carriers in the crystal. We have developed a modification of the shell model in which this screening is included in a consistent way, and we are able to obtain a satisfactory description of the dispersion curves using this model with an ionic charge of 1·34e, and a slightly worse fit using a non-ionic model. Both these models were based on the Hartree dielectric function, and were found to be superior to a model based on the Thomas-Fermi dielectric function.