Pressure-induced densification of vitreous silica: insight from elastic properties

\textit{In situ} high-pressure Brillouin light scattering experiments along loading-unloading paths are used to investigate the compressibility of vitreous silica. An accurate equation of state is obtained below \SI{9}{GPa} using sound velocities corrected for dispersion. Conversely, huge inelastic effects are observed in the range \SIrange{10}{60}{GPa}, unveiling the reversible transformation from the fourfold-coordinated structure to the sixfold one. We find that the associated density changes fully correlate with the average Si coordination number. Decompression curves from above \SI{20}{GPa} reveal abrupt backward coordination changes around \SIrange{10}{15}{GPa} and significant hysteresis. Further, contrary to common wisdom, the residual densification of recovered silica samples can be figured out from the pressure cycles.