Novel phases in the Fe-Si-O system at terapascal pressures

Abstract

The Fe-Si-O ternary system, central to modeling the interiors of terrestrial planets, remains poorly constrained at Terapascal (TPa) pressures characteristic of super-Earth mantles. Using a combination of crystal-structure prediction and ab initio calculations, we identify three ternary compounds stable near 1 TPa: P3 FeSiO4, P3 Fe4Si5O18, and P-3 FeSi2O6. The first two phases are thermodynamically stable at low temperatures, whereas P-3 FeSi2O6 becomes favored above approximately 2000 K. All three are metallic, paramagnetic, and adopt pseudo-binary arrangements derived from the FeO2 and SiO2 end-member structures. Their crystal structures emerge through substitutions of Fe for Si in Fe2P-type SiO2 or of Si for Fe in Pnma-type FeO2, the stable elemental oxides at ~1 TPa. This structural continuity suggests that Fe preferentially substitutes for Si in the canonical Mg-silicates expected at TPa pressures. Notably, these new pseudo-binaries accommodate Fe in six- and nine-fold coordination, in contrast to the eight-fold cubic coordination found in FeO at similar pressures. The thermodynamic conditions under which these phases form from FeO2 and SiO2 mixtures are clarified through quasi-harmonic free-energy calculations. These phases imply a fundamentally different pattern of Fe incorporation into Mg-silicates at TPa pressures compared with that inferred for Earth's mantle, i.e., mainly [Fe]Mg. Their stability may trigger silicate dissociation into oxides ((Mg,Fe)2(Si,Fe)O4 -> 2(Mg,Fe)O + (Si,Fe)O2) at pressures below ~3 TPa, as expected in the Mg-Si-O system, with the extent of dissociation governed by iron content.