Iron oxide motifs and structures from 0.1 to 3 TPa

Iron oxides are fundamental components of planet-forming materials. Understanding the Fe-O system's behavior and properties under pressure can help us identify a plethora of new phases and states possible in exoplanetary interiors, especially terrestrial ones. In this study, we use the adaptive genetic algorithm (AGA) to investigate the structure of iron oxides throughout a wide range of stoichiometries (0.25 < x_O < 0.8) and pressures (P = 0.1, 1.0, and 3.0 TPa). At 0.1 TPa, we successfully identify the experimentally known iron oxides phases. However, conventional DFT incorrectly describes the relative energy ordering; it requires self-consistent DFT+U calculations to correct these phases' energetics. At ultra-high pressures, we find some unreported low-enthalpy Fe_xO_y phases. Combining the previously predicted structures at 0.35 and 0.5 TPa [Weerasinghe et al., JPCM 27, 455501 (2015)], we show that Fe-O compounds form intricate structural motifs at lower pressures. However, at 1 and 3 TPa, most iron oxides adopt simple BCC or BCT motifs. This finding provides a glimpse of iron coordination and oxidation states in more complex phases with additional elements such as Mg, Si, H, etc. at exoplanetary interior pressures, of which we know very little.