1 Introduction

Magnesium oxide (MgO) is one of the most abundant phases in planetary mantles, and understanding its high-pressure behavior is essential for constructing models of the Earth’s and planetary interiors. For a long time, MgO was believed to be among the least polymorphic solids - only the NaCl-type structure has been observed in experiments at pressures up to 227 GPa 173. Static theoretical calculations have proposed that the NaCl-type (B1) MgO would transform into CsCl-type (B2) and the transition pressure is approximately 490 GPa at 0 K (474 GPa with the inclusion of zero-point vibration) 174; 175; 176. Calculations also predicted that MgO remains non-metallic up to extremely high pressure (20.7 TPa) 175; 177, making it to our knowledge the most difficult mineral to metalize. Thermodynamic equilibria in the Mg-O system at 0.1 MPa pressure have been summarized in previous studies 178; 179; 180, concluding that only MgO is a stable composition, though metastable compounds (MgO$_2$, MgO$_4$) can be prepared at very high oxygen fugacities.

Although MgO seems to be so simple and well understood, surprises may be in store for the researcher. For instance, it was shown recently predicted that Xe becomes reactive and forms thermodynamicallystable oxides at the Earth’s mantle 181. In the simple “Li-H" system, in addition to the “normal" LiH, new counterintuitive compounds LiH$_2$, LiH$_6$ and LiH$_8$ were predicted to become stable at pressures above 100 GPa 182, though experiments failed to confirm them so far 183. However, the prediction of new Na-Cl compounds (Na$_3$Cl, Na$_2$Cl, Na$_3$Cl$_2$, NaCl$_3$ and NaCl$_7$) have been confirmed by the experiment 184. If similarly unusual stoichiometries become stable also in the Mg-O system, this could have important chemical and planetological implications. In this paper, we explore this possibility and indeed find two new stoichiometries to be thermodynamically stable at high pressures. These two new stable compounds (MgO$_2$ and Mg$_3$O$_2$) exhibit interesting crystal structures with unusual chemical bonding and insulating and semiconducting electronic structures, respectively.