Antiferromagnetic Ordering and Crystal Field Behavior of NiCl24H2O

Abstract

Magnetic-susceptibility and heat-capacity measurements on single crystals of Ni${\mathrm{Cl}}_2$ · 4${\mathrm{H}}_2$O, which is isomorphic to Mn${\mathrm{Cl}}_2$ · 4${\mathrm{H}}_2$O and Mn${\mathrm{Br}}_2$ · 4${\mathrm{H}}_2$O, indicate an antiferromagnetic-paramagnetic transition at (2.99±0.01) °K. The large zero-field splitting of the ${\mathrm{Ni}}^{2+}$ ion in this material results in an 11 °K doublet-singlet splitting of its ground state with the doublet lying lower. A molecular-field model has been used to obtain an exchange-parameter value of $\frac{\mathrm{zJ}}{k}=-5.25\mathrm{}$ °K. The effect of the crystal field anisotropy on the transition temperature has been interpreted and the results seem to indicate that the molecular-field theory gives a better approximation than do the Green's-function techniques.