Anharmonicity and Phase Diagram of Magnesium Oxide in the Megabar Regime

• Published in: Physical review letters 2020-10, Vol.125 (17), p.1-175701, Article 175701
• Main Authors: Soubiran, François, Militzer, Burkhard
• Format: Article
• Language: English
• Subjects: 3-dimensional systems; 70 PLASMA PHYSICS AND FUSION TECHNOLOGY; Ab initio calculations; Anharmonic lattice dynamics; Anharmonicity; Crystalline systems; Density functional theory; Entropy; First-principles calculations; Hugoniot curves; Intermetallic phases; Lattice dynamics; Liquid phases; Liquid-solid phase transition; Liquids; Magnesium oxide; Molecular dynamics; Phase diagrams; Phase stability; Phase transitions; Single crystal materials; Solid-solid transformations; Superconductors (materials)
• ISSN: 0031-9007; 1079-7114
• DOI: 10.1103/PhysRevLett.125.175701

With density functional molecular dynamics simulations, we computed the phase diagram of MgO from 50 to 2000 GPa up to 20 000 K. Via thermodynamic integration (TDI), we derive the Gibbs free energies of the B1, B2, and liquid phases and determine their phase boundaries. With TDI and a pseudo-quasi-harmonic approach, we show that anharmonic effects are important and stabilize the B1 phase in particular. As a result, the B1-B2 transition boundary in the pressure-temperature plane exhibits a steep slope. We predict the B1-B2-liquid triple point to occur at approximately T = 10 000 K and P = 370 GPa, which is higher in pressure than was inferred with quasiharmonic methods alone. We predict the principal shock Hugoniot curve to enter the B2 phase stability domain but only over a very small range of parameters. This may render it difficult to observe this phase with shock experiments because of kinetic effects.