Melting of B1-Phase MgO From Simultaneous True Radiative Shock Temperature and Sound Speed Measurements to 250 GPa on Samples Preheated to 2300 K

To refine the melting curve, equation of state, and physical properties of MgO we performed plate impact experiments spanning $\sim$170–250 GPa on MgO single crystals, preheated to 2300 K. A controlled thermal gradient in $\sim$20 mm long samples enabled radiative temperature ($\pm$3%–4%) and rarefaction overtake observations (yielding sound speed $\pm$2%) close to the hot Mo driver with a free surface below $\sim$2000 K that minimized evaporation. Ta flyers were launched by two-stage light-gas gun up to 7.6 km/s and sample radiance was recorded with a 6-channel (500–850 nm) pyrometer. Shock front reflectivity was measured at 198 and 243 GPa using $\sim$50/50 sapphire beam-splitters. Most experiments show monotonic increases of shock temperature with pressure, from (168 GPa, 7100 K) to (243 GPa, 9400 K), in good agreement with predictions of our MgO B1 phase equation of state. Measured sound speeds are parallel to but $\sim$10% higher than model predictions for bulk sound speed of solid B1 MgO, confirming ductile behavior of preheated MgO. Two experiments, at 238 and 246 GPa, showed anomalously low shock temperature and sound speed, suggesting melting. Using reported MgO melting data up to 120 GPa and our data at 232–246 GPa, we constructed a maximum-likelihood Simon-Glatzel fit. At Earth's core-mantle boundary pressure (135 GPa), our best-fit interpolated MgO melting point is $T_m=(7.77\pm 0.03)·{10}^3$ K. Our proposed melting line falls within the envelope of theoretical predictions but does not completely agree with any particular model curve. Our results reduce the uncertainty on MgO melting temperature at Earth's core-mantle boundary by a factor of $\sim$17 and provide an anchor for extension to multicomponent systems.