Tetragonal Fe2O: the stable iron oxide at Earth's core conditions

TL;DR

This study identifies Fe2O as a stable iron oxide phase at Earth's core conditions, suggesting oxygen's possible incorporation into the inner core and implications for its composition and density.

Contribution

The paper presents the discovery of Fe2O as a stable phase at high pressures and temperatures, providing new insights into Earth's core composition.

Findings

Fe2O is thermodynamically stable from 200 to 400 GPa.

Fe2O undergoes a hexagonal-to-tetragonal transition under high P-T conditions.

Oxygen can be incorporated into the inner core as Fe2O, matching PREM densities.

Abstract

The Fe-O system is fundamental to understanding the composition and properties of the Earth's core. Recent studies have suggested the possible existence of stable, iron-rich FenO compounds at around 215 GPa. Here, we performed crystal-structure searches and fully anharmonic free-energy calculations to investigate the Fe-FeO system under inner-core conditions. We identified Fe2O as a stable phase and constructed its high P-T phase diagram. Fe2O undergoes a hexagonal-to-tetragonal transition with increasing pressure and temperature. It remains thermodynamically stable against decomposition into Fe and FeO from 200 to 400 GPa and at high temperatures. Although oxygen has been considered nearly absent in the inner core due to its limited solubility, these results suggest that oxygen can, in fact, be incorporated into the solid inner core in the form of an Fe+Fe2O mixture, and can match PREM densities for 53 mol% Fe2O. Our work has the potential to lead to a significant revision of the current understanding of the core's structure and composition.