Decomposition and phase transition of NiS/Ni1−x S in the temperature range 323–973 K

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 Ni3−x S2 is high-temperature heazlewoodite. The Ni3S2 (low-temperature heazlewoodite) and a possible phase, Ni7S6, (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 Ni3−x S2 during quenching from 973 K. The exsolutions of Ni3S2 and Ni7S6 during the quenching are less significant for compositions Ni0.97S and Ni0.92S 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 Ni0.97S, 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.