The Fe-S system was explored in a laser-heated diamond-anvil cell at 89(2) GPa and 2380(120) K to better understand the phase stability of Fe2S. Upon temperature quenching, crystallites of Fe2S were identified, and their structure was investigated using single-crystal X-ray diffraction techniques. At these conditions, Fe2S adopts the C23 structure (anti-PbCl2, Co2P) with space group Pnma (Z = 4). This structure consists of columns of corner-sharing, FeS4 tetrahedra, and columns of edge-sharing FeS5 square pyramids linked along edges in the b direction. Sulfur is in ninefold coordination with Fe. This study marks the first high-pressure structural solution and refinement of Fe2S synthesized in a multigrain Fe+FeS sample at 90 GPa and 2400 K and establishes the stability of C23 Fe2S at these conditions. A previous powder diffraction study reports an orthorhombic Fe2S phase with a C37, Co2Si-like unit cell above 190 GPa. A C23–C37 structural transition is inferred to explain the previously observed unit-cell parameters at higher pressures and temperatures. These results highlight the utility of applying single-crystal X-ray diffraction techniques to high P-T multigrain samples to explore the structural properties of iron-rich phases in Earth and planetary cores.

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