Inelastic Neutron Scattering on Solid and Liquid Tellurium and Selenium

The lattice dynamics of tellurium and selenium are of interest with regard to the relatively strong deformations of the valence electron shells and the asymmetric lattice structure of the crystal which consists of spiral chains in a hexagonal array. Inelastic scattering experiments have been performed on crystalline and liquid selenium and tellurium and on amorphous selenium using a rotating crystal and a triple-axis spectrometer. The experiments on selenium polycrystals and tellurium poly- and single crystals show high-energy modes which can be identified as intra-chain bond stretching modes with a pronounced molecular vibrational character. There are further optical modes at lower energies due to bond-shearing vibrations and to torsional modes of the chains against each other. The results are consistent with infrared experiments. No reliable check of the existing theories has been possible, but a rough agreement was found. Comparing the spectra of polycrystalline, amorphous, and liquid selenium, there is no appreciable shift or broadening of the peaks ascribed to optical intra-chain modes in the amorphous and liquid phase. The influence of small admixtures of iodine has been studied which reduces the length of the chains. For liquid tellurium at 470 °C there are no peaks at all in the spectrum which results from a breakdown of the polymeric structure. The change of the quasi-elastic line width with the chain length was found to be considerably smaller than the change of the reciprocal viscosity. (author)