FeS phase diagram and thermal Equation of State under high pressure

FeS is a key end-member iron alloy relevant to the cores of telluric planets. Here, we present a comprehensive study of the FeS phase diagram up to 150 GPa and 3000 K, using in situ x-ray diffraction measurements in laser-heated diamond anvil cells. Our results extend the determination of the FeS melting curve up to 100 GPa. We have investigated the subsolidus phase relations of FeS, observing transitions between FeS-VI (MnP-type structure) and FeS-IV (distorded NiAs-type structure). We find that this transition is second-order, with the stability range of FeS-IV narrowing with increasing pressure. Additionally, we identify a novel phase, designated FeS-IX, which is formed at high temperatures above the stability field of FeS-IV, and persists up to the melting point of the alloy. By integrating our data with ambient-temperature literature results, we establish a thermal equation of state for FeS over the investigated pressure and temperature range. These findings carry significant implications for understanding the thermal state and structure of planetary cores, with specific relevance to those of Mercury and Mars.