Abstract
Although the phase relations in the Ni–S system have been studied by many researchers, an accurate decomposition temperature range of the near equimolar composition α-NiS/α-Ni1−x
S has not been established due to the complexity brought by the nonstoichiometic nature of α-NiS/α-Ni1−x
S. In the current study, a series of α-NiS/α-Ni1−x
S synthetic schedules were adopted to study the phase relations and variations in cell parameters of various phases in the temperature range 323–643 K. The decomposition of NiS started above 773 K according to the reaction:
where Ni
3−x
S
2 is high-temperature heazlewoodite. The Ni
3S
2 (low-temperature heazlewoodite) and a possible phase, Ni
7S
6, (the uncertainty is due to its low diffraction intensity in XRD profiles) observed in the current study are the products of the instantaneous decomposition of Ni
3−x
S
2 during quenching from 973 K. The exsolutions of Ni
3S
2 and Ni
7S
6 during the quenching are less significant for compositions Ni
0.97S and Ni
0.92S compared with NiS. For equimolar composition α-NiS, the exact transition temperature of α-NiS↔β-NiS (millerite) lies between 623 K (derived from a cooling process) and 670 K (derived from a heating process). For nickel-deficient composition Ni
0.97S, α- and β-phases coexist in the temperature range 573–613 K, and the onset temperature for α- to β-phase transition is lowered by around 50 K (for cooling processes) compared with that of NiS.