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

Magnesium Oxide (MgO) and water (HO) 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 HO can react again at ultrahigh pressure, although Mg(OH) decomposes at low pressure. The reemergent MgO-HO compounds are: MgOH above 400 GPa, MgOH above 600 GPa, and MgOH in the pressure range of 270-600 GPa. Importantly, MgOH 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 MgOH 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.