Prediction of crystal structures and motifs in the Fe-Mg-O system under Earth’s core pressures*

While the crystal structures of Fe-O and Fe-Mg binary systems under pressure have been extensively studied recently, there are still very few studies of the Fe-Mg-O ternary phases at relevant Earth’s core and super-Earth’s mantle pressures. Here, we use the adaptive genetic algorithms (AGA) to search for and study the crystal structure of Fe_xMg_yO_z phases in a wide range of stoichiometries at 200 GPa and 350 GPa. Our results revealed new phases with lower enthalpy than the known compounds in the existing Fe-Mg-O high-pressure structure database. By constructing the convex hull at both pressures, we determine the ground-state phases. We also carry out structure motif analyses for AGA-found stable and metastable structures. We find a wide variety of composition- and pressure-dependent motifs.