Neutron scattering study of the vibrations in vitreous silica and germania

TL;DR

This study extends neutron scattering analysis to include coherent effects, revealing that vibrations in vitreous silica and germania involve tetrahedral rotations and translations, with translational motions exceeding Debye model predictions.

Contribution

It introduces a method to analyze coherent scattering in neutron data and applies it to vitreous silica and germania, providing new insights into their vibrational dynamics.

Findings

Vibrations involve rotation and translation of tetrahedra.

Translational component exceeds Debye model by a factor of two.

Possible link between excess vibrations and phonon broadening.

Abstract

The incoherent approximation for the determination of the vibrational density of states of glasses from inelastic neutron or x-ray scattering data is extended to treat the coherent scattering. The method is applied to new room temperature measurements of vitreous silica and germania on the thermal time-of-flight spectrometer IN4 at the High Flux Reactor in Grenoble. The inelastic dynamic structure factor at the boson peak turns out to be reasonably well described in terms of a mixture of rotation and translation of practically undistorted SiO4 or GeO4 tetrahedra. The translational component exceeds the expectation of the Debye model by a factor of two. A possible relation of this excess to the phonon shift and broadening observed in x-ray Brillouin scattering experiments is discussed.