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 SiO₂ phases, we have found two extraordinary silicon oxides, SiO₃ 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, MgO₃, which becomes stable at 0.89 TPa. We find that not only the (MgO)_x·(SiO₂)_y compounds, but also two (MgO₃)_x·(SiO₃)_y compounds, MgSi₃O₁₂ and MgSiO₆, have stability fields above 2.41 TPa and 2.95 TPa, respectively. The highly oxidized MgSi₃O₁₂ can form in deep mantles of mega-Earths with masses above 20 M_⊕ (M_⊕:Earth’s mass). Furthermore, the dissociation pathways of pPv-MgSiO₃ are also clarified and found to be different at low and high temperatures. The low-temperature pathway is MgSiO₃ ⇒ Mg₂SiO₄ + MgSi₂O₅ ⇒ SiO₂ + Mg₂SiO₄ ⇒ MgO + SiO₂, while the high-temperature pathway is MgSiO₃ ⇒ Mg₂SiO₄ + MgSi₂O₅ ⇒ MgO + MgSi₂O₅ ⇒ MgO + SiO₂. Present results are relevant for models of the internal structure of giant exoplanets and for understanding the high-pressure behavior of materials.