A new high-pressure and high-temperature polymorph of FeS |
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| Authors: | Hiroaki Ohfuji Nagayoshi Sata Hisao Kobayashi Yasuo Ohishi Kei Hirose Tetsuo Irifune |
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| Institution: | (1) Geodynamics Research Center, Ehime University, 2-5 Bunkyo-cho, Matsuyama, Ehime 790-8577, Japan;(2) Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima-cho, Yokosuka, Kanagawa 237-0061, Japan;(3) Graduate School of Material Science, University of Hyogo, 3-2-1 Koto, Kamigori-cho, Ako-gun, Hyogo 678-1297, Japan;(4) Japan Synchrotron Radiation Research Institute, 1-1-1 Koto, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan;(5) Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo 152-8551, Japan |
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| Abstract: | A new polymorph of FeS has been observed at pressures above 30 GPa at 1,300 K by in situ synchrotron X-ray diffraction measurements in a laser-heated diamond anvil cell. It is stable up to, at least, 170 GPa at 1,300 K. The new phase (here called FeS VI) has an orthorhombic unit cell with lattice parameters a = 4.8322 (17) Å, b = 3.0321 (6) Å, and c = 5.0209 (8) Å at 85 GPa and 300 K. Its topological framework is based on the NiAs-type structure as is the case for the other reported polymorphs (FeS I-V). The unit cell of FeS VI is, however, more distorted (contracted) along the 010] direction of the original NiAs-type cell. For example, the c/b axial ratio is ～1.66 at 85 GPa and 300 K, which is considerably smaller than that of orthorhombic FeS II (～1.72) and NiAs-type hexagonal FeS V (=√3 ≈ 1.73). The phase boundary between FeS IV and VI is expected to be located around 30 GPa at 1,300 K. The phase transition is accompanied by gradual and continuous changes in volume and axial ratios and may be second order. At room temperature, FeS VI becomes stable over FeS III at pressures above 36 GPa. It is, therefore, suggested that the phase boundary of FeS III–VI and/or FeS IV–VI has negative pressure dependence. |
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