Paramagnetic Susceptibility of Terbium Hexa-Antipyrene Tri-Iodide

Terbium hexa-antipyrene tri-iodide has a large temperature-dependent susceptibility parallel to the crystal hexagonal symmetry axis and at low temperatures a very much smaller temperature-independent susceptibility perpendicular to this axis. This behavior is interpreted on a hexagonal crystal field model. The parallel susceptibility, which between 1.4 and 150°K is given by ${\chi }_{\mathrm{par}}=8.7\mathrm{}\beta {({H_0}^2+H^2)}^{-\frac{1}{2}}tanh\left[8.7\mathrm{}\beta \frac{{({H_0}^2+H^2)}^{\frac{1}{2}}}{\mathrm{kT}}\right]$ with $H_0=3400\mathrm{}$ Oe, corresponds to a nondegenerate terbium $J_z=\pm 6$ doublet with a zero field splitting of 4°K. The remaining crystal field levels lie more than 150°K above the doublet. The crystal field is found to have strong axial components, with somewhat weaker hexagonal distortions responsible for the doublet splitting. Distortions from hexagonal symmetry, although known to be appreciable in hexa-antipyrene at low temperatures, make only a small contribution to the ${\mathrm{Tb}}^{3+}$ susceptibility.