Iron oxides are fundamental components of planet-forming materials. Understanding the Fe-O system's behavior and properties under high pressure can help us identify many new phases and states possible in exoplanetary interiors, especially terrestrial ones. Using the adaptive genetic algorithm, we investigate the structure of iron oxides for a wide range of stoichiometries $(0.25\le x_{\mathrm{O}}\le 0.8)$ at 1, 2, and 3 TPa. Five ground-state structures with ${\mathrm{Fe}}_2\mathrm{O}$, FeO, ${\mathrm{Fe}}_3{\mathrm{O}}_5$, ${\mathrm{FeO}}_2$, and ${\mathrm{FeO}}_4$ compositions are identified. Phonon calculations confirm their dynamical stability. The ab initio molecular dynamics simulations confirm the thermal stability of Fe-rich phases at high temperatures. The calculated density of states suggests that, except for ${\mathrm{FeO}}_4$, all phases are metallic, but their carrier densities decrease with increasing pressure and oxygen content. The cluster alignment analysis of stable and metastable phases shows that several motifs may coexist in a structure of iron oxides with low O content. In contrast, most iron oxides with high O content adopt a simple bcc motif at TPa pressures. Our results provide a crystal structure database of iron oxides for modeling and understanding the interiors of exoplanets.

• Received 27 October 2021
• Accepted 7 April 2022

DOI:https://doi.org/10.1103/PhysRevMaterials.6.043602