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

Abstract

The energy spectra of monochromatic neutrons scattered on selenium and tellurium 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 atom 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 cm2/s. Its activation energy was about 5 kcallmol.

Structure and relaxation in liquid and amorphous selenium

“…2͒, are in reasonable agreement with experiments on liquid Se. 44,45 At the higher temperatures D can be fitted by an Arrhenius law with an activation energy of 0.3 eV, in agreement with results from first-principle molecular dynamics simulations. 29 In the inset of Fig.…”

High-frequency collective excitations in molten and glassy Te studied by inelastic neutron scattering

“…For such a purpose computer simulations were carried out using a modified version of the MSCAT code. 19 As an initial scattering kernel we have used an estimate for the quasielastic part of the spectra calculated from reported values of the self-diffusion coefficient 15 and using the Sköld approximation. The results turned out to be not very sensitive to the choice of scattering kernel, mostly because of the relatively large neutron-capture cross section of Te which is comparable to the total scattering cross section ͑ abs / scatt ϳ 1.09͒.…”

“…Although several reports on neutron inelastic scattering measurements on the liquid have already appeared in the literature, [14][15][16] their main focus has been the determination of quasielastic scattering widths as well as rough estimates for the spectral frequency distributions. In what follows we will describe our measurements for the liquid sample performed on a thermal triple-axis neutron spectrometer as well as some measurements for a glassy sample that are here taken as a reference.…”

Density variations in liquid tellurium: Roles of rings, chains, and cavities

“…͑4͒͒ and is given for four temperatures in Table II. Measurements of the self-diffusion coefficient in Te have been made using neutron scattering, 44 where the broadening of the elastic peak caused by diffusion of the atoms in a liquid can be related to D, 45 and by observing the motion of radioactive tracer atoms ͑such as Te 127 ͒ in a capillary geometry. 46 The experimental data are characterized by substantial error bars and difficulties in extrapolating values measured at higher temperatures to the melting point.…”