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The phase stability of orthorhombic Fe 2 S was explored to 194 GPa and 2500 K using powder and multigrain synchrotron X‑ray diffraction techniques. Between 30 and 120 GPa, a  C 23-like (Co 2 P,  Pnma ,  Z  = 4) Fe 2 S structure is observed and determined to exhibit a highly compressible  a  axis. A softening of the  a  axis occurs between 120 and 150 GPa and a relative stiffening of the  b  and  c  axes accompanies this compressibility change. Above 150 GPa, the  a  axis stifens as the  b  and  c  axes soften, and a  C 37-like (Co 2 Si,  Pnma ,  Z  = 4) Fe 2 S unit cell is measured. On the basis of these changes in unit cell geometry, a pressure-induced  C 23– C 37 Fe 2 S phase transition is inferred between 120–150 GPa. The  C 23 and  C 37 ( Pnma ,  Z  = 4) structures are closely related and share the same site symmetries. Forming the  C 37 structure from the  C 23 structure requires a shortening of the  a  axis and lengthening of the  b  and  c  axes accompanied by a four- to fivefold coordination change. The softening of the  a  axis above 120 GPa may therefore indicate the onset of a coordination change, and the final compressibility change above 150 GPa may mark the completion of this phase transition. The presented pressure-temperature ( P-T ) stabilities of  C 23 and  C 37 structures of Fe 2 S are in agreement with and resolve the differing observations of two previous studies ( Tateno et al. 2019 ;  Zurkowski et al. 2022 ). As  C 37 Fe 2 S is observed to core-mantle boundary pressures and high temperatures, the  C 37 Fe 2 S density profile through Earth’s outer core was determined by fitting the  C 23 Fe 2 S equation of state (&lt;120 GPa) and applying a 1.6% volume reduction based on the  C 37 Fe 2 S volume residuals to this fit. Comparing the density of liquid  C 37 Fe 2 S with that of liquid hcp-Fe ( Dewaele et al. 2006 ) and the seismologically determined density deficit of Earth’s core ( Irving et al. 2018 ), 13.9 ± 1.5 wt% and 8.6 ± 0.8 wt% sulfur are required to match the densities at the CMB and ICB, respectively, for a purely Fe-S core.

Pressure-induced C 23– C 37 transition and compression behavior of orthorhombic Fe 2 S to Earth’s core pressures and high temperatures