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.