Acoustic velocities and refractive index of SiO2 glass to 57.5 GPa by Brillouin scattering

The hypersonic sound velocity and refractive index of SiO2 glass has been measured to 57.5 GPa at room temperature by Brillouin scattering in diamond cells. On compression, both longitudinal and tranverse modes exhibit an anomalous change in slope in the low-pressure region. Between 12 and 23 GPa, the sound velocities increase rapidly. At higher pressures, the bulk velocity follows a trend similar to that expected for coesite. At 57.5 GPa, the longitudinal velocity of SiO2 glass is 11.85 (0.51) km/s and the transverse velocity is 6.12 (0.06) km/s. The refractive index increases monotonically with pressure and reaches a value of 1.924 (0.081) at 57.5 GPa. It decreases reversibly on decompression to 26 GPa, but displays an irreversible change when decompressed from 16 GPa to ambient pressure. The pressure-density relation calculated from the measured sound velocities within the elastic compression region is in good agreement with several previous determinations.