OPTICAL 2ND-HARMONIC-GENERATION STUDY OF QUARTZ UP TO 31 GPA

Optical second-harmonic-generation (SHG) and Raman scattering experime nts have been performed on quartz up to pressures of 31 GPa. The SHG s ignal and its anisotropy were found to remain approximately constant u p to about 22 GPa at which value the SHG showed a discontinuous decrea se in magnitude, consistent with a phase transition at that pressure. An increase in pressure beyond 22 GPa produced a linear decrease in th e SHG signal and its anisotropy. The observation of SHG above 22 GPa i ndicates that at least one of the crystalline phases of SiO2 stable at those pressures lacks inversion symmetry. A significant SHG signal wa s detected at 31 GPa, indicating that quartz is not entirely amorphous at that pressure with respect to a macroscopic nonlinear dipole momen t.