A mechanism for oxygen concentration change in Fe2O3 and FeO2

Oxygen-rich materials like peroxides and superoxides are often found in alkali metal oxides or alkaline-earth metal oxides due to the small cation to anion radius ratio at ambient. The recently synthesized pyrite-FeO₂ at high pressure suggested a novel stoichiometry in the Fe-O binary system. It is likely to take part in Earth's redox equilibria by storing 33% more oxygen than hematite. Herein, combining in-situ x-ray diffraction experiments and first-principles calculations, we identified that two competing phase transitions starting from Fe₂O₃:1) without sufficient O₂, perosvkite-Fe₂O₃ transits to the post-perovskite structure above 50 GPa; (2) under oxygen-rich condition, O diffuses into the perovskite-type lattice of Fe₂O₃ leading to the pyrite-type FeO₂ phase. We found the O-O bonds in FeO₂ are enforced by external stress and such bonding is only kinetically stable under high pressure. This provides a general mechanism of regassing oxygen to O saturated oxides. Our results also shed light on how O is enriched in mantle minerals under pressure.