The kinetics of the α → β transition in synthetic nickel monosulfide

The kinetic behavior of the α-Ni 1-x S → β-NiS transition was investigated via a series of annealquench experiments using Rietveld quantitative phase analysis of powder X-ray diffraction data. Initial compositions of α-Ni 1-x S 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 Ni 0.97 S, however, E a 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 E a and extent of transition (y) for the initial bulk Ni0.97S was derived using the Refined Avrami method. For Ni-deficient compositions, α-Ni 1-x S, the transformation to β-NiS is accompanied by the exsolution of a progressively more Ni-deficient α-Ni 1-x S and Ni3S4, and the reactions become more sluggish for more metal-deficient compositions.