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

Raymond F. Smith1, Saransh Singh1, Zixuan Ye2, Marius Millot1, Melissa Sims2,*, Anirudh Hari2,3,†, Jon H. Eggert1, Federica Coppari1, and June K. Wicks2

  • *Present address: Department of Physics, University of Alabama, Birmingham, Alabama 35294, USA.
  • †Present address: Stanford University, Department of Materials Science and Engineering, and PULSE Institute, California 94305, USA.
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Phys. Rev. X 16, 041019 – Published 19 October, 2026

DOI: https://doi.org/10.1103/vt45-32bb

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 B1→B2 transformation pathway and subsequent melting behavior. For compression along [111], the shock-temperature evolution exhibits distinct plateaus consistent with equilibrium B1→B2→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.

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Popular Summary

Materials squeezed to extreme pressures and temperatures can change their internal structure in surprising ways, which is important for the behavior of matter inside planets and testing fundamental condensed-matter theories. We investigated MgO, a major building block of rocky planet mantles, subjected to rapid, direction compression. We compressed tiny single crystals of MgO to extreme conditions using shock waves while simultaneously collecting ultrafast x-ray diffraction and optical measurements to track the crystal structure and temperature. The crystal’s orientation impacts the outcome in that MgO transforms with near-equilibrium behavior in one direction but has an 80 GPa delay when compressed in other directions. All orientations converge to complete melting at 572 GPa and 13 200 K, the highest experimentally measured melting temperature of any material. We resolved long-standing discrepancies in the MgO phase diagram and demonstrated the crystal compression direction can control how it transforms. This provides an experimental framework for understanding nonequilibrium phase transitions and improves planetary modeling constraints.

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Phys. Rev. X 16, 041019– Published 19 October, 2026
  • Received 4 March 2026
  • Accepted 18 September 2026
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