α-Nickel sulfate hexahydrate crystals: relationship of growth conditions, crystal structure and properties

Abstract

Studies on α-nickel sulfate hexahydrate (NSH) crystals grown under different conditions are undertaken to investigate how changes in growth conditions affect crystal properties and whether or not there is any modification of the average crystal structure due to changes in crystallization conditions. Thermogravimetric and microhardness studies were carried out on the crystals grown from two different aqueous solutions, one of them containing an excess of sulfuric acid. Raman spectra were recorded and a single-crystal neutron diffraction investigation was conducted on both crystals. A detailed comparison between the two crystal structures and their Raman spectra showed that, although the two crystal structures are very similar, there are slight differences, such as the change in unit-cell volume, differences in the ionic structure, particularly of the sulfate ions, and changes in the hydrogen-bonding network. During solution crystal growth of a salt like NSH, varying the ionic environment around the solute ions influences the interionic interactions between them. Hence it is suggested that the above-mentioned structural differences result from a fine-tuning of the interionic interaction between the cations and anions of NSH in the solution phase. This difference is finally carried over to the crystalline phase. The resulting small crystal structure differences are enough to produce measurable changes in the thermal stability and fragility of the crystals. These differences in crystal properties can be explained on the basis of the observed structural differences between the two crystals grown under different conditions.