Solid–solid reaction, particularly in the Fe–O binary system, has been extensively studied in the past decades because of its various applications in chemistry and materials and earth sciences. The recently synthesized pyrite-FeO₂ at high pressure suggested a novel oxygen-rich stoichiometry that extends the achievable O–Fe ratio in iron oxides by 33%. Although FeO₂ was synthesized from Fe₂O₃ and O₂, the underlying solid reaction mechanism remains unclear. Herein, combining in situ X-ray diffraction experiments and first-principles calculations, we identified that two competing phase transitions starting from Fe₂O₃: (1) without O₂, perovskite-Fe₂O₃ transits to the post-perovskite structure above 50 GPa; (2) if free oxygen is present, 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 formed by the insertion of oxygen into the Pv lattice via the external stress and such O–O bonding is only kinetically stable under high pressure. This may provide a general mechanism of adding extra oxygen to previous known O saturated oxides to produce unconventional stoichiometries. Our results also shed light on how O is enriched in mantle minerals under pressure.

中文翻译：

---

Fe ₂ O ₃和FeO _2中的结构控制氧浓度

固-固反应，特别是在Fe-O二元体系中，由于其在化学，材料和地球科学中的各种应用，因此在过去的几十年中得到了广泛的研究。最近在高压下合成的黄铁矿-FeO ₂提出了一种新颖的富氧化学计量，将可实现的铁氧化物中的O-Fe比值扩大了33％。尽管FeO ₂是由Fe ₂ O ₃和O ₂合成的，但其潜在的固体反应机理仍不清楚。在这里，结合原位X射线衍射实验和第一性原理计算，我们确定了从Fe ₂ O ₃开始的两个竞争相变：（1）没有O ₂，钙钛矿型Fe ₂ O ₃转变为50 GPa以上的钙钛矿后结构；（2）如果存在游离氧，则O扩散到Fe ₂ O ₃的钙钛矿型晶格中，从而导致黄铁矿型FeO ₂相。我们发现，FeO ₂中的O–O键是通过外部应力将氧气插入Pv晶格中形成的，此类O–O键仅在高压下具有动力学稳定性。这可以提供将额外的氧添加到先前已知的O饱和氧化物中以产生非常规化学计量的一般机制。我们的结果也揭示了O在压力下如何富集于地幔矿物中。