Structure and motifs of iron oxides from 1 to 3 TPa

TL;DR

This study explores the high-pressure structures of iron oxides relevant to exoplanet interiors, discovering new phases and analyzing their electronic properties using computational methods across 1 to 3 TPa pressures.

Contribution

It identifies five previously unreported ground-state structures of iron oxides at extreme pressures, expanding the known phase diagram and providing a database for planetary modeling.

Findings

Five new stable iron oxide structures discovered.

Most high-pressure phases are metallic with decreasing carrier density.

Structural motifs vary with oxygen content and pressure.

Abstract

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 (AGA), we investigate the structure of iron oxides for a wide range of stoichiometries ($0.25\leq x_O \leq 0.8$) at 1, 2, and 3 TPa. Five unreported ground-state structures with Fe$_2$O, FeO, Fe$_3$O$_5$, FeO$_2$, and FeO$_4$ compositions are identified. The calculated density of states (DOS) suggests that, except for 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 co-exist 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.