Heat-treatment in sulfur atmosphere can control the defect concentration in crystals in the Ni–S system. In the composition range of Ni_1−xS, two different phases have been demonstrated, one of which takes a disordered structure and the other an ordered structure.
The disordered phase has been found to have the NiAs-type structure with a partial ordering of vacancies. X-ray diffraction patterns of the ordered phase contain superlattice reflections corresponding to the hexagonal cell with dimensions a′=3a₀ and c′=3c₀ and streaks along the c^* axis. Two superstructures belonging respectively to space groups P3₁21 and P3₂21, have been proposed, where vacancies are ordered corresponding to the composition Ni₁₇S₁₈. Electron diffraction investigation indicates that the domain size of each superstructure is larger than about 0.5 μm.
Electric resistivity and magnetic susceptibility at the metal-semiconductor transition for the disordered phase showed hysteresis with tempetature cycling. The transition occurs only in the disordered Ni_1−xS phase, while in the ordered phase all the samples remain metallic at temperatures down to 4.2 K.