Tetragonal : The stable iron oxide at Earth's core conditions
Phys. Rev. B 113, 054107 – Published 6 February, 2026
DOI: https://doi.org/10.1103/j4fd-f7x2
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 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 as a stable phase and constructed its high P-T phase diagram. 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 mixture, and can match PREM densities for 53 mol % . Our work has the potential to lead to a significant revision of the current understanding of the core's structure and composition.
