Novel stable compounds in the Mg-Si-O system under exoplanet pressures and their implications in planetary science

The Mg-Si-O system is the major Earth and rocky planet-forming system. Here, through quantum variable-composition evolutionary structure explorations, we have discovered several unexpected stable binary and ternary compounds in the Mg-Si-O system. Besides the well-known SiO$_{\mathrm{2}}$ phases, we have found two extraordinary silicon oxides, SiO$_{\mathrm{3}}$ and SiO, which become stable at pressures above 0.51 TPa and 1.89 TPa, respectively. In the Mg-O system, we have found one new compound, MgO$_{\mathrm{3}}$, which becomes stable at 0.89 TPa. We find that not only the (MgO)$_{\mathrm{x}}$·(SiO$_{\mathrm{2}})$y compounds, but also two (MgO$_{\mathrm{3}})_{\mathrm{x}}$·(SiO$_{\mathrm{3}})_{\mathrm{y}}$ compounds, MgSi$_{\mathrm{3}}$O$_{\mathrm{12}}$ and MgSiO$_{\mathrm{6}}$, have stability fields above 2.41 TPa and 2.95 TPa, respectively. The highly oxidized MgSi$_{\mathrm{3}}$O$_{\mathrm{12\thinspace }}$can form in deep mantles of mega-Earths with masses above 20 M$_{\mathrm{\oplus }}$ (M$_{\mathrm{\oplus }}$:Earth's mass). Furthermore, the dissociation pathways of pPv-MgSiO$_{\mathrm{3}}$ are also clarified, and found to be different at low and high temperatures. The low-temperature pathway is MgSiO$_{\mathrm{3\thinspace }}\Rightarrow $ Mg$_{\mathrm{2}}$SiO$_{\mathrm{4\thinspace }}+$ MgSi$_{\mathrm{2}}$O$_{\mathrm{5\thinspace }}\Rightarrow $SiO$_{\mathrm{2\thinspace }}+$ Mg$_{\mathrm{2}}$SiO$_{\mathrm{4\thinspace }}\Rightarrow $ MgO $+$ SiO$_{\mathrm{2}}$, while the high-temperature pathway is MgSiO$_{\mathrm{3\thinspace }}\Rightarrow $ Mg$_{\mathrm{2}}$SiO$_{\mathrm{4\thinspace }}+$ MgSi$_{\mathrm{2}}$O$_{\mathrm{5\thinspace }}\Rightarrow $ MgO $+$ MgSi$_{\mathrm{2}}$O$_{\mathrm{5\thinspace }}\Rightarrow $ MgO $+$ SiO$_{\mathrm{2}}$. Present results are relevant for models of the internal structure of giant exoplanets, and for understanding the high-pressure behavior of materials.