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

Abstract

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 italicPatrue3¯‐ Rtrue3¯m‐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.

Magnetism-Vanishing Stabilizes the Pyrite-Type 3d Transition Metal Peroxides at High Pressures

“…To further elucidate the oxidation states of cations, we have calculated the charge transfer of AO 2 compounds with various structural types as well as Fm 3̅ m AO compounds based on Bader analysis (Table S5). ,,,, The calculated charge of Fe and O in FeO 2 is +1.46 and −0.73, respectively, which is in agreement with recent PBE + U and hybrid functional simulations. , The charge of Cr in CrO 2 is predicted to be +1.94, 32.9% larger than that of Fe in FeO 2 . The charge of Mn in MnO 2 is comparable to that of Cr, being +1.92 to +1.96.…”

Structural, magnetic and electronic properties of CrO2 at multimegabar pressures

“…In terms of FeO 2 in the recent study, we have predicted a transition sequence of pyrite-R 3m-I4/mmm at high pressures, different from present calculated results. 53 And the stable region of the R 3m FeO 2 has a span of $1200 GPa whereas the R 3m CrO 2 is energetically unfavorable in the present calculated pressure range. Therefore, this can serve as an indirect evidence that the chemical valence of iron cation is not +4 in FeO 2 under ultra-high pressure.…”

Structure-Controlled Oxygen Concentration in Fe2O3 and FeO2

“…For example, below 40 GPa, Py-FeO 2 starts to degas oxygen and reverts to Fe 2 O 3 . While the thermodynamic phase stability fields for Fe 2 O 3 − and FeO 2 ,, have been extensively investigated, the general structure modification mechanism involving stoichiometric variation remains elusive. The kinetics in the oxygenation reaction is fundamentally important to understand how they are stabilized at high pressure.…”