Stability and anisotropy of (FexNi1-x)2O under high pressure and implications in Earth's and super-Earths' core

Oxygen is thought to be an important light element in Earth's core but the amount of oxygen in Earth's core remains elusive. In addition, iron-rich iron oxides are of great interest and significance in the field of geoscience and condensed matter physics. Here, static calculations based on density functional theory demonstrate that /4/mmm-Fe2O is dynamically and mechanically stable and becomes energetically favorable with respect to the assemblage of hcp-Fe and R3m-FeO above 270 GPa, which indicates that /4/mmm-Fe2O can be a strong candidate phase for stable iron-rich iron oxides at high pressure, perhaps even at high temperature. The elasticity and anisotropy of /4/mmm-(FexNi1-x)(2)O at high pressures are also determined. Based on these results, we have derived the upper limit of oxygen to be 4.3 wt% in Earth's lower outer core. On the other hand, /4/mmm-(FexNi1-x)(2)O with high AV(S) is likely to exist in a super-Earth's or an ocean planet's solid core causing the locally seismic heterogeneity. Our results not only give some clues to explore and synthesize novel iron-rich iron oxides but also shed light on the fundamental information of oxygen in the planetary core.