High-pressure behavior of the Fe–S system and composition of the Earth’s inner core

Abstract

Using the evolutionary crystal structure prediction algorithm USPEX, we identify the compositions and crystal structures of thermodynamically stable compounds in the Fe–S system at pressures in the range of 100–400 GPa. We find that at pressures in the Earth’s solid inner core (330–364 GPa) two compounds are stable—Fe2S and FeS. In equilibrium with iron, only Fe2S can exist in the inner core. Using the equation of state of Fe2S, we find that, in order to reproduce the density of the inner core by adding sulfur alone, 10.6–13.7 mol.% (6.4–8.4 wt.%) sulfur is needed. An analogous calculation for silicon (where the only stable compound at inner core pressures is FeSi) reproduces the density of the inner core with 9.0–11.8 mol.% (4.8–6.3 wt.%) silicon. In both cases, a virtually identical mean atomic mass in the range of 52.6–53.3 results for the inner core, which is much higher than inferred for the inner core from Birch’s law. In the case of oxygen (allowing for the equilibrium coexistence of suboxide Fe2O with iron under core conditions), the inner core density can be explained by the oxygen content of 13.2–17.2 mol.% (4.2–5.6 wt.%), which corresponds to between 49.0 and 50.6. Combining our results and previous work, we arrive at four preferred compositional models of the Earth’s inner core (in mol.%): (i) 86% (Fe+Ni)+14% C; (ii) 84% (Fe+Ni)+16% O; (iii) 84% (Fe+Ni)+7% S+9% H; (iv) 85% (Fe+Ni)+6% Si+9% H.

Fe2S: The Most Fe‐Rich Iron Sulfide at the Earth's Inner Core Pressures

“…When the fitting is made with a reference pressure of 1 bar, we obtain V 0 = 133(1) Å 3 and K 0 = 226(8) GPa with K' 0 = 4 (fixed). They are generally consistent with those from theoretical calculations at 0 K when considering the temperature difference (Bazhanova et al, 2017).…”

“…Finally, 20 peaks from the sample grew and became spotty in a 2D diffraction image in 1 hr (Figures 1b and S1a). All the new peaks fully match those of the orthorhombic phase with Pnma space group (Figure 1c and Table S1), which has been predicted by theory (Bazhanova et al, 2017). Diffraction peaks from SiO 2 were not observed in the present experiments; crystallization of SiO 2 glass is known to be quite sluggish (e.g., Tateno et al, 2015Tateno et al, , 2019Komabayashi et al, 2019).…”

Equations of State and Anisotropy of Fe‐Ni‐Si Alloys

“…Concerning the stability of these phases in the inner core, Ozawa et al (2011) find B2-FeO to be favored over B1-FeO above 240 GPa at 4000 K, thereby suggesting the B2 phase is stable in the inner core, and ab initio studies by Huang et al (2018) Ahrens and Jeanloz (1987) suggest that cubic pyrite is stable up to 320 GPa. While recent theory suggests that C2/m-FeS 2 may be stable at inner core conditions (Bazhanova et al, 2017), experimental equations of state are also not presently available for this phase. In a recent review, Li and Fei (2014) suggest that Fe 7 C 3 is favored over Fe 3 C as the liquidus phase of the Fe-C system at pressures above 7 GPa and therefore favored as a phase in the inner core.…”

Role of temperature and Coulomb correlation in the stabilization of the CsCl-type phase in FeS under pressure

“…However, the chemical composition and crystal structure of the core compounds is still the subject of discussion. Sulfur is seen as one of the most preferred candidates to be present in the core [2] and Fe 1−x S is one of the most widespread sulfides on Earth (encountered also in lunar and meteoric samples [3][4][5][6][7]). Highpressure behavior of iron sulfide (FeS) has been investigated through both high-pressure experiments and ab initio simulations in multiple previous studies [3,5,[8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23].…”

“…The onset of a long-range magnetic order is observed at T N ∼ 600 K. Previous experimental studies demonstrate a series of phase transitions with increasing pressure at room temperature; troilite transforms to a MnP-type structure (FeS II) with the orthorhombic space group P nma above 3.4 GPa [8,24] and further to a monoclinic structure (FeS III) above 6.7 GPa. This transition is accompanied by a lattice volume collapse [4] and a change in the crystal symmetry (space group P 2 1 /a). The structural change from FeS II to III involves abrupt breaking of the long-range magnetic order [10,17,25], spin transition of iron, and metalsemiconductor transition.…”