Optical second-harmonic-generation study of quartz up to 31 GPa

Optical second-harmonic-generation (SHG) and Raman scattering experiments have been performed on quartz up to pressures of 31 GPa. The SHG signal and its anisotropy were found to remain approximately constant up to about 22 GPa at which value the SHG showed a discontinuous decrease in magnitude, consistent with a phase transition at that pressure. An increase in pressure beyond 22 GPa produced a linear decrease in the SHG signal and its anisotropy. The observation of SHG above 22 GPa indicates that at least one of the crystalline phases of ${\mathrm{SiO}}_2$ stable at those pressures lacks inversion symmetry. A significant SHG signal was detected at 31 GPa, indicating that quartz is not entirely amorphous at that pressure with respect to a macroscopic nonlinear dipole moment.