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(n)O compounds has long been speculated. Here, we report that iron reacts with FeO and Fe(2)O(3) 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(n)O compounds (with n > 1) can be formed. Like ε–Fe and B8–FeO, Fe(n)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 Fe(2)O, 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(n)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(n)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.