High-pressure and high-temperature polymorphism of iron sulfide (FeS)

Abstract

The pressure-and temperature-induced phase transitions in FeS have been explored by X-ray diffraction measurements made on single-crystal samples. It is found that the room temperature and pressure structure, troilite (a close relative of the NiAs-type structure), with space group P(~2c and a unit cell of a = v/3A and c ---2C (where A and C refer to an NiAs-type structure and are approximately 3.4 and 5.9A, respectively), transforms at 420 K and ambient pressure, and at 298 K and 3.4 GPa to an MnP-type structure. This structure has space group Pnma and the orthohexagonal unit-cell dimensions a = C, b = ,4 and c -v/3A. For FeS, the deviation of its unit cell from these ideal dimensions is small, and although the transformation results in a triply twinned crystal, the profiles of reflections containing contributions from multiple-twin domains are broadened but not split. A comparison of data from structure refinements adjacent to these transitions shows that, in both cases, the MnP-type phase has an increased distortion of the S atoms from planarity, a less distorted FeS 6 polyhedron, and longer Fe-Fe bonds. The differences in distortion between the structures suggests that their stabilities may depend upon the relative energy contributions from the distorted polyhedron and the Fe-Fe bond. Further increases in temperature produce a second-order transition to an NiAs-type structure, space group P63/mmc, whereas higher pressures cause an increasing sinusoidal distortion of the essentially h.c.p. S atoms. At 298 K and 6.7 GPa another transition takes place to an unknown structure. This transition is accompanied by a 9% volume change and a large decrease in C/A.

Microscopic structure of Fe-Ni and Fe-Ni-S molten alloys of geophysical interest

“…The value corresponding to the first peak compares favorably with that reported for crystalline FeS showing a troilite structure ͑a relative of the NiAs structure with P6 2c space group͒. 26 Its width, which becomes comparable to that of the other two pair correlations, indicates that Fe-S short-ranged correlations last for short times only, thus suggesting that no stable Fe-S entities are formed. An estimate of the position of the first peak for Ni-S correlations ͑not shown͒ gives a value of 2.6 Å.…”

“…25 For such a purpose we have constructed simulation cells containing up to 20 000 atoms with the proportions corresponding to the three compositions. 26 Several starting configurations were chosen. First we chose a random configuration of all the atomic species with the only imposed constraint that the distance between all atoms cannot be less than 1.8 Å.…”

Pressure-Induced High-Spin to Low-Spin Transition in FeS Evidenced by X-Ray Emission Spectroscopy

“…More importantly, FeS falls at the boundary between charge-transfer and Mott-Hubbard insulators in the ZSA phase diagram (∆ < U and U relatively small [6]), so that its electronic structure is far more complicated, and has been very controversial. Pressure-induced structural phase transitions in FeS have been extensively studied, in part because the material is considered to be a major component of the cores of terrestrial planets [7][8][9][10][11][12]. FeS undergoes two structural phase transitions at ambient temperature, from the NiAs-related to a MnP-related structure at 3.5 GPa, and then to a monoclinic phase at 6.5 GPa.…”

Twinning and pseudosymmetry under high pressure

“…[28,29], yet often it is not described whether a significant change of the splitting of reflections has been observed and little or no information about the twin volume fractions as a function of pressure is provided. An exception is the case of Cu 2 CO 3 (OH) 2 [30] where the twin volume fractions were refined as a function of pressure.…”