Iron-rich Fe–O compounds at Earth’s core pressures

Abstract

•A family of Fe-rich FenO compounds is revealed under Earth’s core conditions•The FenO compounds feature close-packed Fe and O monolayers•The separate O monolayer stabilizes FenO compounds at multi-megabar pressures•The Earth’s inner core can contain more oxygen than expected due to FenO phases•It alters our understanding of thermal evolution and seismic features of the core 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 FenO 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 FenO compounds (with n > 1) can be formed. Like ε–Fe and B8–FeO, FenO 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 FenO 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 FenO 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. 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 FenO 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 FenO compounds (with n > 1) can be formed. Like ε–Fe and B8–FeO, FenO 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 FenO 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 FenO 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.

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