Iron-rich Fe-O compounds at Earth's core pressures

Oxygen and iron are the most abundant elements on Earth, and their compounds are key planet-forming components. While oxygen is pervasive in the mantle, its presence in the solid inner core is still debatable. Yet, this issue is critical to understanding the co-evolution and the geomagnetic field generation. Thus far, iron monoxide (FeO) is the only known stoichiometric compound in the Fe-FeO system, and the existence of iron-rich Fe O compounds has long been speculated. Here, we report that iron reacts with FeO and Fe₂O₃ at 220-260 GPa and 3000-3500 K in laser-heated diamond anvil cells. Ab initio structure searches using the adaptive genetic algorithm indicate that a series of stable stoichiometric Fe O compounds (with n > 1) can be formed. Like ε-Fe and B8-FeO, Fe O compounds have close-packed layered structures featuring oxygen-only single layers separated by iron-only layers. Two solid-solution models with compositions close to Fe₂O, the most stable Fe-rich phase identified, explain the X-ray diffraction patterns of the experimental reaction products quenched to room temperature. These results suggest that Fe-rich Fe O compounds with close-packed layered motifs might be stable under inner core conditions. Future studies of the elastic, rheological, and thermal transport properties of these more anisotropic Fe O solids should provide new insights into the seismic features of the inner core, inner core formation process and composition, and the thermal evolution of the planet.