Structures and phase equilibria of FeS under high pressure and temperature

Abstract

High-pressure behavior of FeS up to 16 GPa and 800°C is investigated by energy-dispersive type X ray powder diffraction method. Four phases (the troilife phase, MnP-related phase, NiAs type phase, and a high-pressure phase) are observed in this pressure and temperature range. The high-pressure phase at 10.30 GPa and 27°C, the structure of which has been unknown, can be indexed on a monoclinic system: α=8.044(3)A, b=5.611(2)A, c=6.433 (4)A, β=93.11(4)°, and Z=12. The high pressure phase is calculated to be 12% denser than the troilite phase at ambient conditions. All these four phases are found to have an NiAs-type framework in their structures. Phase equilibria of FeS are determined by paying attention to diffraction lines with d-spacings larger than 3A, due to a long-range order of the NiAs-type framework. The phase boundary between the MnP-related phase and the simple NiAs-type phase can be described by a straight line (P/GPa = 0.0212 × T/°C - 3.38) in the pressure and temperature range up to 11GPa and 700°C. The result shows that high-pressure behavior of the simple NiAs-type phase may play an important role in the interior of the terrestrial planets.