Ultrahigh-Pressure Phase Transitions in FeS2 and FeO2: Implications for Super-Earths' Deep Interior

Iron oxides play an important role in planetary evolution, but little information about the structural properties of AX2- type iron oxides under extreme conditions limits our understanding of the so- called super- Earth planets. Here an investigation into the high- pressure behavior of FeO2 as well as its sulfide counterpart FeS2, has been conducted by first- principle calculations based on density functional theory. Both FeO2 and FeS2 are predicted to undergo a Pa3- R3m- I4/ mmm transition sequence at extreme pressure. Differences in high- pressure behaviors between FeO2 and FeS2 have been discussed in detail, such as effective coordination number and elasticity. This study not only resolves the controversy about the structural stability of FeS2 under high pressure but also suggests the tenfold coordinated I4/ mmm- type FeO2 may contribute to local chemical heterogeneity by accumulation in the deep interior of a large super- Earth or water- rich planet, whose pressure at the core- mantle boundary can reach 2 TPa.