Positive sound dispersion in vitreous GeO2 at high pressure

We determined the density fluctuations spectrum [S(Q,omega)] of vitreous and polycrystalline GeO2 by inelastic x-ray scattering in the wave vector (Q) range between 3 and 13 nm(-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 >= 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.