Structural prediction of Fe-Mg-O compounds at Super-Earth's pressures

TL;DR

This study uses crystal structure prediction to identify stable Fe-Mg-O phases at pressures up to 3 TPa, revealing a preference for BCC motifs and the impact of oxygen enrichment, thus extending knowledge relevant to super-Earth interiors.

Contribution

It systematically predicts new stable Fe-Mg-O phases at ultra-high pressures using the adaptive genetic algorithm, including insights into their structural motifs and composition effects.

Findings

Fe-Mg-O system favors BCC motifs under ultra-high pressures

Oxygen enrichment lowers phase enthalpies

Several unreported stable phases predicted up to 3 TPa

Abstract

Terrestrial exoplanets are of great interest for being simultaneously similar to and different from Earth. Their compositions are likely comparable to those of solar-terrestrial objects, but their internal pressures and temperatures can vary significantly with their masses/sizes. The most abundant non-volatile elements are O, Mg, Si, Fe, Al, and Ca, and there has been much recent progress in understanding the nature of magnesium silicates up to and beyond ~3 TPa. However, a critical element, Fe, has yet to be systematically included in materials discovery studies of potential terrestrial planet-forming phases at ultra-high pressures. Here, using the adaptive genetic algorithm (AGA) crystal structure prediction method, we predict several unreported stable crystalline phases in the binary Fe-Mg and ternary Fe-Mg-O systems up to pressures of 3 TPa. The analysis of the local packing motifs of the low-enthalpy Fe-Mg-O phases reveals that the Fe-Mg-O system favors a BCC motif under ultra-high pressures regardless of chemical composition. Besides, oxygen enrichment is conducive to lowering the enthalpies of the Fe-Mg-O phases. Our results extend the current knowledge of structural information of the Fe-Mg-O system to exoplanet pressures.