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

Oxygen is the most abundant element on Earth. While pervasive in the mantle, its presence in the core is still a subject of debate but critical to our understanding of the core-mantle co-evolution and 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 (Fe_nO with n > 1) has long been questioned. Here we report that iron reacts with FeO and Fe₂O₃ at 220-260 GPa and 3,000-3,500 K in laser-heated diamond-anvil cells. Using the adaptive genetic algorithm, we find the reaction products consist of a series of Fe_nO stoichiometric compounds and solid solutions (e.g., Fe₂₅O₁₃ and Fe₂₈O₁₄) whose X-ray diffraction patterns agree well with Le Bail refinements of the experimental reaction products. Like ε-Fe, Fe_nO compounds have a typical hexagonal close-packed layered structure, featuring oxygen-only layers between iron layers. Our results suggest that Fe-rich Fe_nO compounds with unique physical properties become stable under Earth's solid inner core conditions.