Positive sound dispersion in vitreousGeO2at high pressure

Abstract

We determined the density fluctuations spectrum $[S(Q,\ensuremath{\omega})]$ of vitreous and polycrystalline ${\text{GeO}}_{2}$ by inelastic x-ray scattering in the wave vector ($Q$) range between 3 and $13\phantom{\rule{0.28em}{0ex}}{\mathrm{nm}}^{\ensuremath{-}1}$ at pressures ($P$) between ambient and 7.1 GPa. The polycrystalline sample displays the expected $P$ evolution of the $Q$-dispersion relation of longitudinal acoustic (LA) modes. On the contrary, the dispersion relation of the glassy sample for $P\ensuremath{\ge}1$ GPa shows an excess energy of the LA modes, while at ambient pressure follows the trend expected from ultrasonic and light scattering data. These findings point towards a pressure-induced positive sound dispersion, with an associated high-frequency sound velocity comparable to that of the polycrystalline sample at the same $P$.