The kinetic behavior of the α-Ni1−xS → β-NiS transition was investigated via a series of anneal-quench experiments using Rietveld quantitative phase analysis of powder X-ray diffraction data. Initial compositions of α-Ni1−xS were found to play an important role in the kinetics of the transition. The activation energy (Ea) for this α- to β-phase transition is 16.0 (±0.5) kJ/mol for NiS in the temperature range 343 to 423 K, and 13.0 (±0.5) kJ/mol in the temperature range 523 to 623 K. For Ni0.97S, however, Ea decreases from 73.0 (±0.5) to 17.0 (±0.5) kJ/mol over the course of the reaction in the temperature range 573 to 593 K. The relationship between Ea and extent of transition (y) for the initial bulk Ni0.97S was derived using the Refined Avrami method. For Ni-deficient compositions, α-Ni1−xS, the transformation to β-NiS is accompanied by the exsolution of a progressively more Ni-deficient α-Ni1−xS and Ni3S4, and the reactions become more sluggish for more metal-deficient compositions.

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