Hugoniot, sound speed, and phase transitions of single-crystal sapphire for pressures 0.2–2.1 TPa

Sapphire (${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$) is a major constituent of the Earth's mantle and has significant contributions to the field of high-pressure physics. Constraining its Hugoniot over a wide pressure range and identifying the location of shock-driven phase transitions allows for development of a multiphase equation of state and enables its use as an impedance-matching standard in shock physics experiments. Here, we present measurements of the principal Hugoniot and sound velocity from direct impact experiments using magnetically launched flyers on the Z machine at Sandia National Laboratories. The Hugoniot was constrained for pressures from 0.2--2.1 TPa and a four-segment piecewise linear shock-velocity--particle-velocity fit was determined. First-principles molecular dynamics simulations were conducted and agree well with the experimental Hugoniot. Sound-speed measurements identified the onset of melt between 450 and 530 GPa, and the Hugoniot fit refined the onset to $525\ifmmode\pm\else\textpm\fi{}13$ GPa. A phase diagram which incorporates literature diamond-anvil cell data and melting measurements is presented.