Iron-rich Fe-O compounds with closest-packed layers at core pressures

TL;DR

This study reports the discovery of new iron-oxygen compounds under core-like pressures, revealing their layered structures and electronic properties, which could influence our understanding of Earth's inner core composition.

Contribution

The paper presents experimental identification and theoretical analysis of novel Fe-O compounds with layered close-packed structures at extreme pressures, previously unreported.

Findings

Identified Fe$_n$O compounds with layered close-packed structures.

Revealed electronic properties relevant to Earth's inner core.

Demonstrated chemical reactions of Fe with FeO at high pressures.

Abstract

Oxygen solubility in solid iron is extremely low, even at high pressures and temperatures. Thus far, no Fe-O compounds between Fe and FeO endmembers have been reported experimentally. We observed chemical reactions of Fe with FeO or Fe$_2$O$_3$ $in\ situ$ x-ray diffraction experiments at 220-260 GPa and 3,000-3,500 K. The refined diffraction patterns are consistent with a series of Fe$_n$O (n $>$ 1) compounds (e.g., Fe$_{25}$O$_{13}$ and Fe$_{28}$O$_{14}$) identified using the adaptive genetic algorithm. Like $\epsilon$-Fe in the hexagonal close-packed (hcp) structure, the structures of Fe$_n$O compounds consist of oxygen-only close-packed monolayers distributed between iron-only layers. $Ab\ initio$ calculations show systematic electronic properties of these compounds that have ramifications for the physical properties of Earth's inner core.