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

Abstract

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.