Refractive index and phase transformation of sapphire under shock pressures up to 210GPa

Under shock pressures up to 210 GPa, we measured the refractive index of sapphire at a wavelength of 1550 nm by performing plate impact experiments in order to investigate its refractive-index change behaviors and phase transitions along the Hugoniot state. There were two discontinuities in the refractive index at similar to 65 to 92 GPa and similar to 144 to 163 GPa, respectively. Moreover, above the Hugoniot elastic limit, the pressure dependence of the refractive index was divided into three segments, and there were large differences in their pressure-change trends: the refractive index decreased evidently with pressure in the first segment (similar to 20 to 65 GPa), remained nearly constant from similar to 92 to similar to 144 GPa in the second segment, and obviously increased with pressure in the last segment (similar to 163 to 210 GPa). Our first-principles calculations suggest that the observed discontinuities were closely related to the corundum-Rh2O3(II) and Rh2O3(II)-CaIrO3 structural transitions, and the shock-induced vacancy point defects could be one factor causing these great discrepancies in pressure-change trends. This work provides sapphire refractive-index information in a megabar-pressure range and clear evidence of its shock structural transitions. This not only has a great significance for the velocity correction of laser interferometer experiments and the analysis of sapphire high-pressure properties but also indicates a possible approach to explore the shock transitions of transparent materials. Published by AIP Publishing.