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We explore the structural properties of Mg, MgO, and MgSiO3 liquids from ab initio computer simulations at conditions that are relevant for the interiors of giant planets, stars, shock compression measurements, and inertial confinement fusion experiments. Using path-integral Monte Carlo and density functional theory molecular dynamics, we derive the equation of state of magnesium-rich liquids in the regime of condensed and warm dense matter, with densities ranging from 0.32 to 86.11 g cm−3 and temperatures from 20,000 K to 5 × 108 K. We study the electronic structure of magnesium as a function of density and temperature and the correlations of the atomic motion, finding an unexpected local maximum in the pair correlation functions that emerges at high densities which decreases the coordination number of elemental magnesium and reveals a higher packing. This phenomenon is not observed in other magnesium liquids, which maintain a rather constant coordination number. 

Investigating water worlds presents a unique opportunity to understand the fundamental processes of planetary formation and evolution. One key aspect is characterizing the interactions between water and rock under the pressures and temperatures present within these worlds. Investigating the conditions for the homogeneous mixing of these materials is imperative to characterizing bulk properties and evolution of water‐rich exoplanets. Here we use density functional molecular dynamics simulations to study MgO‐H₂O mixtures at high pressure–temperature conditions where H₂O occurs in solid, superionic, or liquid form. MgO, the representative rocky material, can be either solid or liquid. We start from 500 K at 120 GPa, increasing the temperature step by step up to 8000 K. By inspection, we determine the temperature at which MgO‐HO homogeneously mix in our simulations. At 6000 K and 174 GPa is when we find the system to homogeneously mix. This heat‐until‐it‐mixes approach provides us with an upper bound on the temperature for the mixing of MgO and H₂O. We find that homogeneous mixing occurs at sufficiently low temperatures to be relevant for the collisional growth of a water‐rich planet.

 

Abstract We perform ab initio simulations of beryllium (Be) and magnesium oxide (MgO) at megabar pressures and compare their structural and thermodynamic properties. We make a detailed comparison of our two recently derived phase diagrams of Be (Wu et al 2021 Phys. Rev. B 104 014103) and MgO (Soubiran and Militzer 2020 Phys. Rev. Lett. 125 175701) using the thermodynamic integration technique, as they exhibit striking similarities regarding their shape. We explore whether the Lindemann criterion can explain the melting temperatures of these materials through the calculation of the Debye temperature at high pressure. From our free energy calculations, we find that the melting line of both materials is well represented by the Simon–Glazel fit T m ( P ) = T 0 (1 + P / a ) 1/ c , where T 0 = 1564 K, a = 15.8037 GPa and c = 2.4154 for Be, while T 0 = 3010 K, a = 10.5797 GPa and c = 2.8683 for the MgO in the B1. For the B2 phase, we use the values a = 26.1163 GPa and c = 2.2426. Both materials exhibit negative Clapeyron slopes on the boundaries between the two solid phases that are strongly affected by anharmonic effects, which also influence the location of the solid–solid–liquid triple point. We find that the quasi-harmonic approximation underestimates the stability range of the low-pressure phases, namely hcp for Be and B1 for MgO. We also compute the phonon dispersion relations at low and high pressure for each of the phases of these materials, and also explore how the phonon density of states is modified by temperature. Finally, we derive secondary shock Hugoniot curves in addition to the principal Hugoniot curve for both materials, and study their offsets in pressure between solid and liquid branches.