Second-order phase transition of FeS under high pressure and temperature Original Research Article

A temperature-induced second-order phase transition of FeS was observed at 4.6 GPa. The NiAs type phase was stable at 500°C, and it transformed to its super-structured phase with a hexagonal cell near-by 420°C in a temperature cooling process. An abnormal temperature behavior of the c-axis causes an unusually low thermal expansion of FeS in this pressure range, and they can be explained by a relaxation of a repulsion along the c-axis in the hexagonal phase.