The investigation of atomic motions in crystalline, amorphous and liquid selenium, and in crystalline and liquid tellurium by neutron spectroscopy

The energy spectra of monochromatic neutrons scattered on selenium and telllurium have been investigated by means of a rotating crystal spectrometer. In polycrystalline selenium the highest spectral line can be identified as an intrachain bond stretching mode. Transforming selenium from its crystalline to its amorphous and liquid state, the shift of the line energy is only a few percent. The other lines in the spectra (from bond shearing and torsional modes of the chains) are influenced more strongly. For tellurium, on the other hand, the spectral lines of the solid disappear completely in the liquid phase. The experimental results are compared with the results of optical spectroscopy. Furthermore, the diffusive motion of the atoms has been studied by quasi-elastic neutron scattering for liquid selenium containing various admixtures of iodine (0–6 %), which change the average chain length of the polymeric melt, and for liquid tellurium. For Se, one obtains an effective diffusion constant D which describes the motion of small chain segments over atomistic distances. D is of the order of 10–5 cm2/s. Its activation energy was about 5 Kcal/mol.