Thermodynamics of MgO shocked to 250 GPa and 9000~K

Plate impact experiments in the 200-250 GPa pressure range were done on $<$100$>$ single-crystal MgO preheated before compression to 1850 K. Hot Mo(driver)-MgO targets were impacted with Ta flyers launched by the Caltech two-stage light-gas gun up to 7.5 km/s. Radiative temperatures and shock velocities were measured with 3-5\% and 1-2\% respective uncertainty by a 6-channel pyrometer with 3 ns time resolution, over 500-900 nm spectral range. MgO shock front reflectivity was determined in additional experiments at 220 and 250 GPa using $_{\verb1~1}$50/50 high-temperature sapphire beamsplitters. Shock temperatures and preheated MgO Hugoniot data reported here are in good agreement with the corresponding values calculated using Mie-Gr\"{u}neisen equation of state with $\gamma_0$=1.4 and constant $\gamma/V$. Our experiments showed no evidence of MgO melting up to 250 GPa and 9.2 kK. The highest shock temperatures exceed the extrapolated melting curve of Zerr \& Boehler by $>$3000 K at 250 GPa, which seems too much for any realistic superheating.