Phase stabilities and spin transitions of Fe₃(S_1-xP_x) at high pressure and its implications in meteorites

Fe-S-P compounds have been observed in many meteorites and could be the important components in planetary cores. Here we investigated the phase stability of Fe-3(S,P) solid solutions and synthesized high-quality Fe3(Si_ 13x) high-pressure phases in the multi-anvil press. The physical properties of Fe-3(S0.5130.5) were further studied in the diamond-anvil cell by synchrotron X-ray diffraction and emission spectroscopy. The solubility of S in the Fe-3(S,P) solid solution increases with increasing pressure. The minimum pressure to synthesize the pure Fe3S and Fe-3(S0.13P0.87) is about 21 and 8 GPa, respectively. The observed discontinuity in unit-cell parameters at about 18 GPa is caused by the high-spin to low spin transition of iron, supported by X-ray emission spectroscopy data. The sulfur solubility in Fe-3(S,P) solid solutions could be an excellent pressure indicator if such solid solutions are found in nature.