HIGH PRESSURE PHASE TRANSITIONS OF FeS. Keiji Kusaba1, Yasuhiko Syono1, Takumi Kikegawa2, Osamu Shimomura2, 1IMR, Tohoku Univ., Sendai 980-77, Japan, 2PF, KEK, Tsukuba 305, Japan
In-situ observation of FeS by an energy-dispersive type X-ray method shows that a high pressure phase at 10.30GPa and 27deg.C has a monoclinic cell, and that the high pressure phase still has a framework of the NiAs type structure.
Troilite type FeS was synthesized in a sealed glass tube at 690deg.C for two weeks and then 100deg.C for two weeks. From the d-spacing of the 102 diffraction line of the NiAs type structure, the atomic ratio in troilite was calculated to be Fe:S=49.95:50.05. High pressure and high temperature experiments up to 16GPa and 800deg.C were carried out using a DIA type cubic anvil apparatus 'MAX80' installed at AR-NE5C in KEK. Diffraction lines with d-spacings larger than 3Å were carefully observed for a phase determination.
The phase boundary between a high pressure phase with the MnP type structure or a hexagonal structure (Fei et al. 1995) and simple NiAs type phases was determined by a simple straight line with a positive slope in the present P-T conditions. The phase boundary between the troilite and the high pressure phase has a negative slope. A complex diffraction pattern was observed at 27deg.C and above 7GPa. From twenty-six diffraction lines of the high pressure phase at 10.30GPa and 27deg.C, an automatic indexing computer code 'DICVOL' showed that there was no possible crystal system higher than orthorhombic symmetry, and that there were only two possibilities of a monoclinic system. Comparing these two possibilities, a reasonable unitcell for the high pressure phase can be proposed to be a = 8.044(4)Å, b = 5.611(2)Å, c = 6.433(3)Å, ß = 93.11(4)deg. and Z = 12. The volume of the high pressure phase is calculated to be 2.8% smaller than that of the Fei's phase at 11.17GPa and 300deg.C.