Temperature dependence of the zero-field splitting of Ni2+ in ZnSiF6 ⋅ 6H2O and ZnSiF6 ⋅ 6D2O at low temperatures

EPR measurements at 9.25 GHz were made on single crystals of Ni2+ in ZnSiF6 ⋅ 6H2O and ZnSiF6 ⋅ 6D2O at small temperature intervals from 4 K to above 100 K. Least-squares fits of the zero-field splitting data below 70 K for the two crystals to a theoretical model of Shrivastava, based on the interaction in the long wavelength approximation, yielded values for the Debye temperature θD of 130 K for ZnSiF6 ⋅ 6H2O and 99 K for ZnSiF6 ⋅ 6D2O. The value of θD for the hydrated salt is roughly 30% larger than the more direct estimate of Vasyukov et al. from measurements of the stress coefficients. The deviations from the theoretical curves for both crystals are appreciable above 75 K.