High-Pressure Behavior of Iron Sulfide
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platinum-wire heater fitted around the protruding portion of the piston-cylinder and a small molybdenum-wire heater positioned around the diamond anvils. This double-heater hightemperature diamond-anvil cell is capable of achieving pressures greater than 125 GPa at temperatures up to 1100 K in a mildly reducing atmosphere (Ar with 1% H2 ) [12].
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DATA COLLECTION
Fig. 1. Experimental configuration. The design of the diamond-anvil cell is similar to that of Mao et al. [10]. High temperatures were achieved by using a double-heater system. Temperatures were measured with thermocouples (TCl and TC2). Energy-dispersive X-ray diffraction techniques were used for data acquisition. The polycrystalline iron sulfide sample was loaded into a sample chamber, 200 Rm in diameter by 70 gm in thickness, drilled from a preindented rhenium gasket. Only one-third of the chamber volume was filled with sample. Small ruby grains and gold foil were placed in the sample chamber as pressure calibrants. The sample chamber was then filled with neon gas at 200 MPa in a high-pressure gas-loading device [13] and subsequently sealed at pressures about 1 GPa in a hydrostatic neon pressure medium. The pressure gradient across the sample chamber is minimized under neon pressure medium environment, especially at high temperature. Pressures were determined by measuring the lattice parameter of gold, the internal standard, based on its PV-T equations of state [14]. Temperatures were measured with a Pt/Pt- 10%Rh thermocouple placed near the sample chamber (Fig. 1). The experiments were conducted at the xl7c beamline, the National Synchrotron Light Source, Brookhaven National Laboratory. Polychromatic (white) wiggler synchrotron x-radiation was used for energy-dispersive X-ray diffraction measurements. A highly collimated X-ray beam, regulated by two mutually perpendicular slits, was aligned with the detection system and the center of the sample chamber in the diamond-anvil cell. The diffraction data were collected with an intrinsic germanium solid-state detector in 4096 channels at a fixed 20 angle. The energy-channel number relationship was calibrated by measuring the energies of well-determined
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X-ray emission lines (KaI and Kp) of Mn, Cu, Rb, Mo, Ag, Ba, and Th. The 20 angle was calibrated by measuring the energies of diffraction peaks, corresponding to the known interplanar spacings dhkl, of gold at ambient conditions. A typical energy-dispersive X-ray diffraction spectrum of FeS was obtained in about 10 minutes. RESULTS AND DISCUSSION Over 500 energy-dispersive X-ray diffraction spectra of FeS were collected in the pressure range of 0-25 GPa and temperature range of 300-900 K. Phase transitions were determined by in situ measurements. Figure 2 shows the experimentally determined phase diagram of FeS. At 300 K, we confirmed two previously observed phase transitions. Troilite (FeS I), NiAs-type structure with a (4l3a,2c) unit cell, transforms to a MnP-type structure (FeS II) at about 3.4 GPa
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