Shock-induced optical emission and high-pressure phase transformation of sapphire

Emission from sapphire window material was measured by spontaneous spectroscopic and multi-wavelength pyrometer techniques. The spectral distribution as a function of wavelength fit well to the grey-body spectrum. We analyzed the emissions and discovered that they arise from shear banding, which is a typical thermal radiation. It was found that the color temperature of shocked sapphire changes linearly and regularly with stress increases from 40 GPa to 59 GPa but becomes monotonic for stresses from 87 GPa to 120 GPa. The change in behavior indicates that a shock-induced phase transformation occurs between 59 GPa and 87 GPa.