Room-temperature densification ofa-SiO2versus pressure

Using Brillouin scattering, we have investigated the room-temperature densification of hydrostatically compressed a-${\mathrm{SiO}}_2$. Irreversible densification occurs in the range 10–25 GPa; at higher pressures (25–40 GPa) no further densification is observed. The densified state is found to be metastable and decreases in density over time spans of the order of years. The presence of large uniaxial stresses during compression is found to lead to a densified state which relaxes much more rapidly (∼ hours) towards the undensified state.