Prediction of crystal structures and motifs in the Fe-Mg-O system at Earth's core pressures

TL;DR

This study uses an adaptive genetic algorithm to predict stable Fe-Mg-O crystal structures at Earth's core pressures, revealing new phases and insights into their stability and motifs relevant to planetary interiors.

Contribution

It introduces newly discovered stable phases in the Fe-Mg-O system at high pressures and constructs the ternary convex hull, advancing understanding of Earth's core composition.

Findings

Discovered three new stable Fe-Mg-O phases at 350 GPa.

Constructed the Fe-Mg-O ternary convex hull.

Indicated potential stabilization of BCC iron by O or Mg at core pressures.

Abstract

Fe, Mg, and O are among the most abundant elements in terrestrial planets. While the behavior of the Fe-O, Mg-O, and Fe-Mg binary systems under pressure have been investigated, there are still very few studies of the Fe-Mg-O ternary system at relevant Earth's core and super-Earth's mantle pressures. Here, we use the adaptive genetic algorithm (AGA) to study ternary Fe$_x$Mg$_y$O$_z$ phases in a wide range of stoichiometries at 200 GPa and 350 GPa. We discovered three dynamically stable phases with stoichiometries FeMg$_2$O$_4$, Fe$_2$MgO$_4$, and FeMg$_3$O$_4$ with lower enthalpy than any known combination of Fe-Mg-O high-pressure compounds at 350 GPa. With the discovery of these phases, we construct the Fe-Mg-O ternary convex hull. We further clarify the composition- and pressure-dependence of structural motifs with the analysis of the AGA-found stable and metastable structures. Analysis of binary and ternary stable phases suggest that O, Mg, or both could stabilize a BCC iron alloy at inner core pressures.