The two well-studied forms of nickel monosulfides are β-NiS (millerite) and α-NiS. β-NiS, a trigonal/rhombohedral structure crystal, is naturally formed as a low temperature hydrothermal or alteration product of other nickel minerals. β-NiS is also called millerite after W. H. Miller, the crystallographer who introduced the system of crystallogrphic indices. Compared with α-NiS, β-NiS is always referred to as the low-temperature form of NiS (< 652 K). It has a high metallic conductivity and is speculated to possess a diamagnetic structure (Krishnakumar et al., 2002). Millerite is the only form of NiS found in nature and it is of metallurgical interest, as it is often associated with other important nickel-iron or copper-iron ores (Power, 1981; Hubli et al., 1995; Legrand et al., 1998). Millerite generally occurs in fine hairlike or capillary crystals, which give it the synonyms: “hair pyrite” and “capillary pyrite”. β-NiS has a near equimolar composition NiS, whereas α-NiS has a variable stoichiometry range, α-Ni1-xS (Grønvold and Stølen, 1999). The maximum x value (in α-Ni1-xS) varies with temperature, and has not been precisely determined (Grønvold and Stølen, 1999; Stølen et al., 1994; Rau, 1975; Kullerud and Yund, 1962).
The high temperature phase, α-NiS/α-Ni1-xS, has the NiAs-type hexagonal structure, is quenchable and remains a metastable phase under ambient conditions.
There have been numerous studies on this metastable α-NiS/α-Ni1-xS at low temperatures due to the distinct metal→ semiconductor transition (MS transition) of α-NiS at 265 K, below which a small band gap occurs and the gap progressively increases with decreasing temperature (Sparks and Komoto, 1963; Nakamura et al., 1994; Okamura et al., 1999). α-NiS/α-Ni1-xS also plays an important role in studies of the crystal chemistry and phase relations in metal sulfides systems because of its relationship with other economically important Ni bearing sulfide minerals, and especially because α-NiS and pyrrhotite are structurally related so that α-NiS can be considered as a compositional Ni end-member of nickeliferous pyrrhotite, (Ni,Fe)1-xS (Legrand et al., 1998; Nesbitt and Reinke, 1999). The NiAs structure of α-NiS exhibits a paramagnetic-antiferromagnetic transition at –8 oC (Néel temperature). The sublattice magnetization goes from zero to full saturation on cooling through –8oC. Previous powder diffraction results and the measurements of magnetic and electrical properties through the transition indicate that the metal→ semiconductor transition of α-NiS is first-order (Sparks and Komoto, 1963, 1967, 1968).
In the antiferromagnetic state, the moments of Ni in a hexagonal layer are coupled ferromagnetically within (001) planes and antiferromagnetically between adjacent (001) planes. This makes the magnetic unit cell of α-NiS identical to the crystal unit cell (Trahan and Goodrich, 1970). The metal→ semiconductor transition temperature (TMS) is a strong function of composition, dropping from –8 oC for α-NiS to –198 oC for α-Ni0.97S (Sparks and Komoto, 1968). This transition temperature (TMS) also is affected by pressure, decreasing rapidly with increasing pressure (Anzai and Ozawa, 1968; McWhan et al., 1972). Nakamura and Fujimori (1993) reported the opposite effects by introducing small amount of Ni and Co impurities to “adjust” the TMS. Ni increases TMS whereas Co decreases TMS. There is some disagreement about the structures of α-phase below and
above the metal→ semiconductor transition temperature. Many researchers believe there is no crystal structure modification of the α-phase during the metal→ semiconductor transition. However, Trahan and Goodrich (1970) discussed a possible subtle change in the lattice symmetry during the MS transition of α-NiS/α-Ni1-xS, from P63/mmc above TMS to P63mc below TMS.
Upon heating, the low-temperature stable phase (millerite) transforms to the high-temperature form, α-NiS, at 379 oC (Kullerud and Yund, 1962). This transition temperature (Tβ-α) was measured at ambient pressure. Sowa et al. (2004) reported the phenomenon of a strong pressure dependence of Tβ-α. The temperature for the transition from millerite to NiAs-type NiS decreases dramatically with increasing pressure. Owing to the higher compressibility of β-NiS (millerite) compared with that of the α-phase, the NiAs-type structure is believed to be unstable at high pressures. The coordination of Ni changes from 5 (in β-NiS structure) to 6 (in α-NiS structure) during the β- to α-phase transition (shown in Figure 2.6). This β- to α-phase is a reconstructive transition, involving breaking and rearranging Ni-S bonds. As the compositional range of the α-phase is wider than that of the β-phase, the equilibrium compositions of α- and β- phases depend on whether the transition is from α- to β-phase or from β- to α-phase. For the transition from β-NiS to α-NiS, all the β-phase transforms into the α-phase with no variations in composition. For the transition from a nickel deficient α- Ni1-xS to β-NiS, the exsolution of more nickel rich β-NiS from the nickel deficient α- Ni1-xS host render
the α-phase host even more deficient in Ni. The present study attempts to reconcile the ambiguities regarding the stoichiometry effect on the transition kinetics of the α- to β-phase transition at ambient pressure.
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