We performed a series of experiments in the system Fe-FeS at a pressure of 21 GPa and temperatures between 950 and 1400 °C, and we found two new iron-excess iron-sulfur compounds, Fe3S and Fe2S, formed at subsolidus temperatures. Powder X-ray diffraction data revealed that Fe3S has a tetragonal cell, isostructural with Fe3P (space group I4̅). The tetragonal unit-cell dimensions for Fe3S are a = 9.144(2) Å and c = 4.509(2) Å, with a zero-pressure density of 7.033 g/cm3. Static compression experiments on Fe3S were carried out in a diamond-anvil cell, using synchrotron X-ray diffraction technique. A least-squares fit to the experimental data at room temperature yielded bulk modulus K0 = 170 ± 8 GPa with a corresponding pressure derivative K0′ = 2.6 ± 0.5 or K0 = 150 ± 2 GPa with fixed K0′ = 4.

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