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

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 around 1 TPa: P 3 FeSiO 4 , P 3 Fe 4 Si 5 O 18 , and P 3 ¯ FeSi 2 O 6 . The first two phases are thermodynamically stable at low temperatures, whereas P 3 ¯ FeSi₂O₆ becomes favored above ∼2000 K. All three are metallic, paramagnetic, and adopt pseudo-binary arrangements derived from the FeO 2 and SiO 2 end-member structures. Their crystal structures emerge through substitutions of Fe for Si in Fe 2 P-type SiO 2 or of Si for Fe in Pnma -type FeO 2 , the stable elemental oxides around 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 FeO 2 and SiO 2 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)O 4 → 2(Mg,Fe)O + (Si,Fe)O 2 ) at pressures lower than ∼3 TPa, with the extent governed by iron content.