Sound velocities and refractive index of densified a-${\mathrm{SiO}}_2$ to 25 GPa

Using Brillouin scattering we have measured the two independent elastic moduli of a-${\mathrm{SiO}}_2$ with increasing and decreasing pressures up to 25 GPa. For descending pressures the shear constant shows irreversible behavior similar to that previously reported for the longitudinal mode. Although the bulk modulus at a given pressure can be evaluated from the measured elastic constants, the irreversible changes which occur on densification preclude the possibility of evaluating the density as a function of pressure by an integration of the compressibility. An estimate of the density is obtained indirectly from the measured refractive index.