Anharmonicity and Phase Diagram of Magnesium Oxide in the Megabar Regime
With density functional molecular dynamics simulations, we computed the phase diagram of MgO from 50 to 2000 GPa up to 20000 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= 10000 K and P = 370 GPa, which is higher in pressure than was inferred with quasi-harmonic 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. Here, this may render it difficult to observe this phase with shock experiments because of kinetic effects.
- Research Organization:
- Univ. of California, San Diego, La Jolla, CA (United States)
- Sponsoring Organization:
- USDOE National Nuclear Security Administration (NNSA); National Science Foundation (NSF)
- Grant/Contract Number:
- NA0003842; SC0016248
- OSTI ID:
- 1784846
- Journal Information:
- Physical Review Letters, Vol. 125, Issue 17; ISSN 0031-9007
- Publisher:
- American Physical Society (APS)Copyright Statement
- Country of Publication:
- United States
- Language:
- English
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Anharmonic lattice dynamics
Entropy
Lattice dynamics
Liquid-solid phase transition
Phase diagrams
Phase transitions
Solid-solid transformations
3-dimensional systems
Crystalline systems
Liquids
Single crystal materials
Ab initio calculations
Density functional theory
First-principles calculations
Molecular dynamics