  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.
