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

Jin Liu; Yang Sun; Chaojia Lv; Feng Zhang; Suyu Fu; Vitali B Prakapenka; Caizhuang Wang; Kaiming Ho; Jungfu Lin; Renata M Wentzcovitch

doi:10.1016/j.xinn.2022.100354

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

Innovation (Camb). 2022 Nov 15;4(1):100354. doi: 10.1016/j.xinn.2022.100354. eCollection 2023 Jan 30.

Authors

Jin Liu^{1

2}, Yang Sun³, Chaojia Lv¹, Feng Zhang⁴, Suyu Fu⁵, Vitali B Prakapenka⁶, Caizhuang Wang⁴, Kaiming Ho⁴, Jungfu Lin⁵, Renata M Wentzcovitch^{3

7

8}

Affiliations

¹ Center for High Pressure Science and Technology Advanced Research, Beijing 100094, China.
² CAS Center for Excellence in Deep Earth Science, Guangzhou 510640, China.
³ Department of Applied Physics and Applied Mathematics, Columbia University, New York, NY 10027, USA.
⁴ Department of Physics, Iowa State University, Ames, IA 50011, USA.
⁵ Department of Geological Sciences, Jackson School of Geosciences, The University of Texas at Austin, Austin, TX 78712, USA.
⁶ Center for Advanced Radiation Sources, University of Chicago, Chicago, IL 60439, USA.
⁷ Department of Earth and Environmental Sciences, Columbia University, New York, NY 10027, USA.
⁸ Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964, USA.

Abstract

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 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 _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 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 _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.