Magnesium oxide-water compounds at megabar pressure and implications on planetary interiors

Abstract

Magnesium Oxide (MgO) and water (H₂O) are abundant in the interior of planets. Their properties, and in particular their interaction, significantly affect the planet interior structure and thermal evolution. Here, using crystal structure predictions and ab initio molecular dynamics simulations, we find that MgO and H₂O can react again at ultrahigh pressure, although Mg(OH)₂ decomposes at low pressure. The reemergent MgO-H₂O compounds are: Mg₂O₃H₂ above 400 GPa, MgO₃H₄ above 600 GPa, and MgO₄H₆ in the pressure range of 270-600 GPa. Importantly, MgO₄H₆ contains 57.3 wt % of water, which is a much higher water content than any reported hydrous mineral. Our results suggest that a substantial amount of water can be stored in MgO rock in the deep interiors of Earth to Neptune mass planets. Based on molecular dynamics simulations we show that these three compounds exhibit superionic behavior at the pressure-temperature conditions as in the interiors of Uranus and Neptune. Moreover, the water-rich compound MgO₄H₆ could be stable inside the early Earth and therefore may serve as a possible early Earth water reservoir. Our findings, in the poorly explored megabar pressure regime, provide constraints for interior and evolution models of wet planets in our solar system and beyond.

Fig. 1. Formation enthalpy and crystal structure of MgO–H₂O compounds.

a Convex hull of formation enthalpy at 300–900 GPa. The circle symbols represent stable compositions. The x symbols represent the compositions that decompose. The zero-point energy of phonons is considered in this figure. For MgO, we consider its B1 and B2 phases. For water, we consider all the stable phases mentioned in Ref. including ice X, Pbcm, Pbca, and P3₁21 phase. b The formation enthalpy of MgO₄H₆ at 250–300 GPa. With ZPE, MgO₄H₆ becomes stable at 270 GPa. c–e The crystal structure of Mg₂O₃H₂, MgO₃H₄, and MgO₄H₆, respectively. The orange, red and white spheres correspond to Mg, O, and H atoms.

Fig. 2. The dynamic properties of MgO–H₂O compounds from molecular dynamics simulations.

a The mean square displacement (MSD) of Mg₂O₃H₂ at 400 GPa, 6000 K (the isentropic condition of Uranus). b The MSD of MgO₃H₄ at 700 GPa, 5000 K (the isentropic condition of Neptune). c The MSD of MgO₄H₆ at 500 GPa, 5000 K (the isentropic condition of Uranus). d–f The snapshots of MD trajectories. The orange and red spheres correspond to Mg and O atoms. The blue points represent H atoms. The position of H atoms from 1000 frames is shown in the picture. We use magenta spheres to highlight the trajectory of a single H atom.

Fig. 3. The phase diagrams of MgO–H₂O compounds.

a Mg₂O₃H₂, b MgO₃H₄ and c MgO₄H₆. The purple and blue lines represent the isentropic lines of Uranus and Neptune. The solid and dashed purple line corresponds to U1 and U2 models. The solid and dashed blue line corresponds to N1 and N2b models. The data of models are from ref. . The diamonds represent the condition of the core–mantle boundary.

Fig. 4. Analysis for superionic behavior in MgO–H₂O compounds.

a The projected mean square displacement (MSD) of Mg₂O₃H₂ at 400 GPa, 6000 K. b The projected MSD of MgO₃H₄ at 700 GPa, 5000 K. c The projected MSD of MgO₄H₆ at 500 GPa, 5000 K. d–f Spatial distribution of protons in Mg₂O₃H₂, MgO₃H₄, and MgO₄H₆ at 1000 K. We use M to denote the midpoint of O_aO_b. The radial distance represents the length of HM in Å. The angle represents the angle formed by O_aO_b and HM.

Fig. 5. The transport properties of the MgO–H₂O compounds (a-b).

a, b Green, red, blue, and black lines represent the MgO₄H₆, the MgO₃H₄, the Mg₂O₃H₂, and the MgO at 600 GPa, respectively. The green, red, and blue dashed lines represent the phase boundary of the MgO₄H₆, MgO₃H_4, and Mg₂O₃H₂, respectively. κ means heat conductivity and σ means electronic conductivity.

Fig. 6. Suggested interior models for internal structure models of Uranus and Neptune based on the data from the U2 and N1 models in Ref. .

The blue-brown gradual change region is where MgO–H₂O compounds can exist.