Structure type and bulk modulus of Fe3S, a new iron-sulfur compound

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.