Orientation-Dependent Structural Phase Transitions and Melting in Shock-Compressed MgO

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Abstract
Magnesium oxide (MgO) is a prototypical strongly bonded ionic solid and an expected major constituent of rocky planet mantles and gas giant cores, yet its high-pressure phase diagram and shock response remain incompletely understood. Here we combine laser-driven decaying-shock and steady-shock compression with nanosecond x-ray diffraction, velocimetry, and optical pyrometry to investigate phase transitions in single-crystal MgO compressed to 600 GPa and 13 000 K. We observe a pronounced crystallographic orientation dependence in the B 1 → B 2 transformation pathway and subsequent melting behavior. For compression along [111], the shock-temperature evolution exhibits distinct plateaus consistent with equilibrium B 1 → B 2 → liquid transitions. In contrast, compression along [110] produces a single temperature discontinuity and diffraction signatures indicative of B2 nucleation from a transient disordered layer at the shock front. The combined thermodynamic and structural data indicate that near-instantaneous disordering can mediate stress relaxation under rapid uniaxial compression. Above 572 ± 24 GPa , all orientations converge to complete melting at 13 200 ± 600 K , constraining the B2-liquid boundary at multimegabar pressures and identifying MgO as the highest-temperature solid measured at planetary interior conditions. These results demonstrate that crystallographic orientation governs phase-transition pathways in strongly compressed ionic solids and provide a benchmark for modeling matter under extreme planetary interior conditions.
Funding Information
  • U.S. Department of Energy
  • Johns Hopkins University
  • Lawrence Livermore National Laboratory (DE-AC52-07NA27344)
  • Lawrence Livermore National Laboratory (15-ERD-014)
  • Lawrence Livermore National Laboratory (17-ERD-014)
  • Lawrence Livermore National Laboratory (20-ERD-044)
  • Lawrence Livermore National Laboratory (27-ERD-013)
  • National Laser Users’ Facility Program (DE-NA0003933)
  • Department of the Defense (HDTRA1-20-2-0001)
  • High Energy Density Science Center

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