Magnetic Properties of NiSO4·7H2O and α-NiSO4·6H2O at Low Temperatures

The magnetic susceptibilities of single crystals of NiSO 4 ·7H 2 O and α-NiSO 4 ·6H 2 O have been measured in the temperature range between liquid helium and room temperatures. The magnetic susceptibility of these salts at low temperatures shows that the level splittings in the ground of each salt are not so small. The seventh water molecule in the heptahydrate salt forms a hydrogen bond and the hexahydrate salt has not a hydrogen bond. The signs of the isotropic Weiss constant of both salts which have be obtained from the measurements of magnetic susceptibilities are opposite to each together. They are θ A V =0.15±0.2°C, g a = g b = g c =2.22±0.02 for NiSO 4 ·7H 2 O and θ A V =-0.5±0.2°C, g a = g b =2.22±0.02 for α-NiSO 4 ·6H 2 O from the expression χ= C /( T -θ). It has been shown that symmetry of crystalline field for NiSO 4 ·7 2 O is rhombic whereas that of α-NiSO 4 ·6H 2 O is tetragonal. The values of the ground level splitting and exchange interaction for NiSO 4 ·7H 2 O that are given from the present magnetic studies are respectively, | D - E |=2.0±0.2 cm -1 , 2| E |=3.0±0.2 cm -1 and A =-0.28±0.2 cm -1 . These values for a α-NiSO 4 ·6H 2 O are respectively, | D |=4±0.3 cm -1 , A =0.4±0.2 cm -1 . The angle between the tetragonal crystalline field axes and the crystallographic c axes in α-NiSO 4 ·6H 2 O is 33° from the present analysis.