Second-Order Phase Transition of FeS under High Pressure and Temperature

High-pressure and high-temperature behavior of FeS was investigated up to 16 GPa and 750°C . A temperature-induced phase transition was observed at 4.6 GPa. The NiAs type phase was stable above 420°C, and it transformed to its super-structured phase with a hexagonal cell below 420°C in a temperature cooling process. The phase transition could be explained as a second-order transition, based on the analysis of X-ray diffraction intensities from the super-lattice.