Ultrahigh‐Pressure Phase Transitions in FeS 2 and FeO 2 : Implications for Super‐Earths' Deep Interior

Iron oxides play an important role in planetary evolution, but little information about the structural properties of AX 2 ‐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 FeO 2 as well as its sulfide counterpart FeS 2 , has been conducted by first‐principle calculations based on density functional theory. Both FeO 2 and FeS 2 are predicted to undergo a Pa 3 ¯ ‐ R 3 ¯ m ‐ I 4/ mmm transition sequence at extreme pressure. Differences in high‐pressure behaviors between FeO 2 and FeS 2 have been discussed in detail, such as effective coordination number and elasticity. This study not only resolves the controversy about the structural stability of FeS 2 under high pressure but also suggests the tenfold coordinated I 4/ mmm ‐type FeO 2 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.