Experimental constraints on the B1-B2 transition and B2-melt curve of MgO

Magnesium oxide (MgO), a key component of rocky planet mantles, has been extensively studied due to its importance in understanding planetary evolution and dynamics. Two critical properties of MgO are: (i) its melting curve under pressure and (ii) the pressure at which it transitions from the rocksalt (B1) phase to the CsCl (B2) phase. However, experimental data to accurately determine these properties remain limited. In this study, we used laser-driven uniaxial shock compression along three high-symmetry crystal axes: [100], [110] and [111]. Our dataset includes shock-decay and in-situ X-ray diffraction data for steady shocks in MgO, revealing strong anisotropy during the B1→B2 phase transition and the onset of B2→liquid pressures. Complete melting was observed at approximately 575 GPa and 13.2 kK, marking the highest Hugoniot melting temperature measurement for a solid to date. For samples oriented along the [110] and [100] axes, the B1→B2 transition occurs near 400 GPa. However, compression along the [111] axis results in a lower B2 onset pressure of around 340 GPa. Our findings also reveal unique high strain-rate deformation behaviors, such as the formation of supercooled liquid states at the shock front and orientation-dependent phase transformation pathways. For the first time, our results resolve significant discrepancies between theoretical predictions and experimental measurements, providing the most accurate constraints on MgO's melting curve. Beyond their implications for planetary science, these measurements challenge our conventional understanding of material behavior under extreme strain-rate compression.