The lattice dynamics of thallous bromide

We have measured the frequencies of a representative set of normal modes of vibration of a single crystal of thallous bromide at room temperature and at 100°K using inelastic neutron scattering techniques. Most of the frequencies are almost independent of temperature, but some modes seem to display marked broadening of the neutron groups at the higher temperature. Various rigid-ion and shell models have been fitted to the results obtained at 100°K. A shell model involving 14 adjustable parameters is reasonably successful in describing the dispersion curves, although some discrepancies remain which are best revealed by an analysis utilizing Rosenstock’s sum rule. Extensive calculations have been made of the properties of the best model and for comparison some of the properties of other models have also been calculated.