Heat Capacity of α-NiSO4·6H2O between 1 and 20°K. Electronic Energy Levels of the Ni++ Ion

The heat capacity of α-NiSO4·6H2O has been measured over the temperature range between 1 and 20°K. The heat capacity passes through a rounded maximum of 1.51 cal deg—1 mole—1 at 2.58°K. The magnetic contribution to the heat capacity is well fitted by that calculated for the three spin states associated with the singly degenerate ground orbital level of the Ni++. From the heat-capacity data the energy spacing between the lowest and middle spin states is calculated to be 4.48±0.07 cm—1 and between the lowest and highest spin states 5.05±0.07 cm—1. The magnetic susceptibilities in the directions of the three principal axes of a Ni++ ion are calculated and, by comparison with literature data on the magnetic anisotropy and single-crystal magnetic susceptibilities, the orientations of these axes relative to the crystal axes are deduced. Data of other investigators on the magneto-optical rotation and high field magnetization are interpreted.