The vibrational motions in vitreous silica at high temperatures

Vitreous silica was studied at the temperatures 293 K, 673 K and 1073 K using inelastic neutron scattering. The zeroeth moment of the dynamic scattering function S( Q, E) agrees well with neutron diffraction results. It was found that the structure factors for elastic and inelastic scattering change with temperature in a Q region which reveals mostly long-range atomic interactions rather than those from nearest and next-nearest neighbours. The temperature variation was interpreted as if the structural and dynamical correlations between neighbouring SiO 4 polyhedra decrease when the temperature increases. Effective Debye-Waller factors were derived and found to increase linearly with temperature. It was also found that the vibrational amplitudes of the atoms in silica, within the experimental errors, are the same as those in high cristobalite. Through extrapolation to zero wave vector transfer of the spectrum of the longitudinal current-current correlation function, effective frequency distribution functions were obtained as well as generalized structure factors for inelastic scattering.