  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.

Materials Science