In this work, hydrated nickel sulfates (NSH) were obtained spontaneously by storing nanocrystalline nickel sulfide composites (NiS2-NiS) under ambient conditions for several months. The samples, which contained different proportions of NiS2-NiS nanophases, were prepared by mechanochemistry by simply ball-milling Ni and S elements at a Ni34S66 composition in the absence of a solvent. X-ray powder diffraction (XRPD) was used to follow the evolution of the phase transitions of the milled samples over approximately 5 years. A few weeks after the grinding process, the monoclinic NiSO4 · 6 (H2O) phase is already detected. Rietveld analyzes showed that the NiS2 and NiS phase fractions decrease over time and that more than 90% nickel sulfate hexahydrate is present in the end product for the sample with a meal of 24 hours. The phases with a small crystallite size (~ 20 nm) and the high micro-elongation of the nickel sulfide nanophases favor this process, which suggests that the sulfate nucleation occurs in disordered regions, also called interface components, and that the growth rates are determined by the exposure time to the sample the ambient atmosphere. DSC analysis shows at least one phase transition, dehydration, and the presence of sulfur in some aged samples. The magnetic measurements at room temperature show a mixed magnetism for the fresh samples and a paramagnetic behavior for the aged ones. We have also observed that the growth of the NSH phases can be accelerated by keeping the samples at high humidity and reaching more than 80% of the NSH phases after 35 days. The chemical reactions during milling and aging and a preliminary model to explain this spontaneous compositional transition from nickel sulfides to nickel sulfate hydrates are presented.

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