[Paper Review] Prediction of novel stable compounds in the Mg-Si-O system under exoplanet pressures

Haiyang Niu, Artem R. Oganov+2 more|arXiv (Cornell University)|Oct 11, 2015

High-pressure geophysics and materials34 references3 citations

TL;DR

Using quantum variable-composition evolutionary structure searches, this study predicts novel stable Mg-Si-O compounds under extreme pressures relevant to mega-exoplanets. Key findings include the stability of SiO3 (above 0.51 TPa), SiO (above 1.89 TPa), MgO3 (above 0.89 TPa), and highly oxidized MgSi3O12 and MgSiO6 above 2.41 and 2.95 TPa, respectively, indicating potential for complex oxide mantles in super-Earths.

ABSTRACT

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.

Motivation & Objective

• To predict stable compounds in the Mg-Si-O system under extreme pressures relevant to rocky exoplanets.
• To understand the high-pressure phase stability of silicate and oxide materials in planetary interiors.
• To explore the formation of highly oxidized compounds such as MgSi3O12 in deep mantles of massive exoplanets.
• To clarify the dissociation pathways of pPv-MgSiO3 under varying temperature conditions.
• To provide materials data for improving internal structure models of giant exoplanets.

Proposed method

• Employed quantum variable-composition evolutionary structure search (XC-ES) to explore stable phases across the Mg-Si-O composition space.
• Used density functional theory (DFT) calculations to evaluate formation energies and thermodynamic stability at high pressures.
• Systematically explored binary and ternary compounds in the Mg-Si-O system at pressures up to 3 TPa.
• Identified stable phases by comparing formation energies and phonon stability across different compositions.
• Tracked phase transitions and dissociation pathways of MgSiO3 under low- and high-temperature conditions.
• Validated stability using energy decomposition and thermodynamic analysis under extreme conditions.

Experimental results

Research questions

• RQ1 What novel stable compounds form in the Mg-Si-O system under pressures exceeding 1 TPa, typical of super-Earth interiors?
• RQ2 At what pressure do highly oxidized phases like SiO3, SiO, and MgO3 become thermodynamically stable?
• RQ3 Can compounds such as MgSi3O12 and MgSiO6 form in deep mantles of massive exoplanets with pressures above 2.4 TPa?
• RQ4 How do the dissociation pathways of pPv-MgSiO3 differ under low-temperature versus high-temperature conditions?
• RQ5 What implications do these new phases have for the internal structure and mineralogy of mega-Earths?

Key findings

• SiO3 becomes stable above 0.51 TPa, representing a novel high-pressure silicon oxide phase.
• SiO is predicted to be stable at pressures exceeding 1.89 TPa, indicating a new stoichiometry in silicon oxides.
• MgO3 forms as a stable compound at pressures above 0.89 TPa, expanding the known stoichiometry of magnesium oxides.
• MgSi3O12, a highly oxidized magnesium silicate, is stable above 2.41 TPa, suggesting potential formation in super-Earth mantles.
• MgSiO6 is stable above 2.95 TPa, indicating the existence of complex, high-oxidation-state silicates at extreme pressures.
• The dissociation of pPv-MgSiO3 proceeds via distinct low- and high-temperature pathways, with different intermediate phases.