Probing vibrational modes in silica glass using inelastic neutron scattering with mass contrast

Abstract

The effective vibrational density of states (VDOS) has been derived from inelastic neutron-scattering data, for isotopically substituted $\text{Si}\text{ }{^{18}\text{O}}_{2}$ and $\text{Si}\text{ }{^{16}\text{O}}_{2}$ glasses, to gain information about the relative contribution to the Si and O partial VDOS. This is a necessary point of comparison for vibrational mode analyses of molecular-dynamics models. The mass contrast has led to a measurable shift between vibrational mode frequencies in the effective VDOS of $\text{Si}\text{ }{^{18}\text{O}}_{2}$ and $\text{Si}\text{ }{^{16}\text{O}}_{2}$, which is well reproduced in an ab initio simulation. The vibrational band centered at 100.2 meV is confirmed to have significantly lower contribution from the oxygen partial VDOS, than the higher (150.3 and 135.8 meV) and lower energy bands (53.3 meV).