Unexpected Stoichiometries in Mg-O System under High Pressure

TL;DR

This study uses ab initio simulations to discover unexpected stable magnesium-oxygen compounds at high pressures, revealing new chemical behaviors and potential planetary relevance.

Contribution

It identifies two novel stable Mg-O compounds, MgO2 and Mg3O2, at high pressures, expanding understanding of magnesium oxides under extreme conditions.

Findings

MgO2 becomes stable at 116 GPa

Mg3O2 becomes stable at 500 GPa

Mg3O2 exhibits electride characteristics

Abstract

Using ab initio evolutionary simulations, we explore all the possible stoichiometries for Mg-O system at pressures up to 850 GPa. In addition to MgO, our calculations find that two extraordinary compounds MgO2 and Mg3O2 become thermodynamically stable at 116 GPa and 500 GPa, respectively. Detailed chemical bonding analysis shows large charge transfer in all magnesium oxides. MgO2 contains peroxide ions [O-O]2-, while non-nuclear electron density maxima play the role of anions in the electride compound Mg3O2. The latter compound is calculated to have a much narrower band gap compared to MgO and MgO2. We discuss conditions at which MgO2 and Mg3O2 might exist in planetary conditions.