Prediction of novel stable compounds in the Mg-Si-O system under exoplanet pressures

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(2) phases, we have found two extraordinary silicon oxides, SiO(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(3), which becomes stable at 0.89 TPa. We find that not only the (MgO)(x)·(SiO(2))(y) compounds, but also two (MgO(3))(x)·(SiO(3))(y) compounds, MgSi(3)O(12) and MgSiO(6), have stability fields above 2.41 TPa and 2.95 TPa, respectively. The highly oxidized MgSi(3)O(12) can form in deep mantles of mega-Earths with masses above 20 M(⊕) (M(⊕):Earth’s mass). Furthermore, the dissociation pathways of pPv-MgSiO(3) are also clarified, and found to be different at low and high temperatures. The low-temperature pathway is MgSiO(3) ⇒ Mg(2)SiO(4) + MgSi(2)O(5) ⇒ SiO(2) + Mg(2)SiO(4) ⇒ MgO + SiO(2), while the high-temperature pathway is MgSiO(3) ⇒ Mg(2)SiO(4) + MgSi(2)O(5) ⇒ MgO + MgSi(2)O(5) ⇒ MgO + SiO(2). Present results are relevant for models of the internal structure of giant exoplanets, and for understanding the high-pressure behavior of materials.