Frequency-dependent equation of state of fused silica to 10 GPa

Using a new technique for optically determining strain in the diamond cell, we have measured the static compression of fused silica and a Ca-Mg-Na glass to over 10 GPa. In our experiments, irreversible compaction of fused silica is precipitated by shear stresses above 10 GPa. For the Ca-Mg-Na glass we observe irreversible compaction beyond 8 GPa under hydrostatic loading. In both glasses we find that the bulk modulus increases sharply at high hydrostatic pressures (\ensuremath{\sim}11 and \ensuremath{\sim}7 GPa, respectively). We show that this increase in bulk modulus can be explained in terms of the transition between relaxed and unrelaxed moduli that is observed at zero pressure, low temperatures, and higher frequencies. Our compression data, measured under quasistatic (\ensuremath{\sim}${10}^{\mathrm{\ensuremath{-}}4}$ Hz) conditions, are consistent with a wide range of acoustic absorption (\ensuremath{\sim}${10}^{5}$--${10}^{10}$ Hz) and dielectric loss ${(10}^{3}$--${10}^{4}$ Hz) measurements in constraining the activation volume for this relaxation process to be ${V}^{\mathrm{*}}$=7.9\ifmmode\pm\else\textpm\fi{}1.7 ${\mathrm{cm}}^{3}$/mol. We propose that the relaxation involves a change in the compression mechanism, from Si---O---Si bond bending to Si---O bond compression induced either by low temperatures or by high pressure.