Crystal-Field Effects in CoCl2·6H2O

The heat capacities of Co${\mathrm{Cl}}_2$·6${\mathrm{H}}_2$O and Ni${\mathrm{Cl}}_2$·6${\mathrm{H}}_2$O have been measured between 13 and 200°K. The heat capacity of the Co${\mathrm{Cl}}_2$·6${\mathrm{H}}_2$O lattice has been estimated from the data on the ${\mathrm{Ni}}^{++}$ salt using a corresponding-states argument and subtracted from the measured total. The remainder is a Schottky anomaly due to the presence of excited Kramers doublets within several hundred ${\mathrm{cm}}^{-1\mathrm{}}$ of the lowest one. The doublet separations and $g$ factors of the ${\mathrm{Co}}^{++}$ ion in a crystalline field containing cubic, tetragonal, and rhombic components have been computed as functions of the field parameters and spin-orbit coupling constant. A consistent interpretation of the thermal data and the principal values of the $g$ tensor ($g_1=2.23\mathrm{}$, $g_2=5.13\mathrm{}$, $g_3=4.90\mathrm{}$) deduced from susceptibility measurements is found to require that the crystalline field contain substantial tetragonal and rhombic components. There is an indication that the spin-orbit coupling constant is smaller than the free-ion value.