5 Conclusions

In summary, we performed a systematic search for possible stoichiometries in the Mg-O system at pressures up to 850 GPa. Other than the well-known compound, MgO, we found that two more stoichiometries (MgO$_2$ and Mg$_3$O$_2$) become thermodynamically stable at pressures above 116 GPa and 500 GPa, respectively. Our analysis reveals that bonding in both insulating $t$-MgO$_2$ and semiconduting $t$-Mg$_3$O$_2$ exhibits significantly ionic character. MgO$_2$ is stabilized by the formation of the peroxide [O-O]$^{2-}$ anion, while the strong electron localization in Mg octahedron plays the role of an additional anion and makes the anion-deficient compound Mg$_3$O$_2$ stable. These two compounds might exist in interiors of other planets, and are thus potentially important for our fundamental understanding of the universe - in addition to presenting new striking chemical phenomena.