Pair Spectra and Magnetic Properties of Ni 2 + in Double-Nitrate Crystals

The properties of isolated ${\mathrm{Ni}}^{2+}$ ions in the two divalent sites ($X$ and $Y$) of the double-nitrate crystals have been determined at 77 and 4.2 K for ${\mathrm{La}}_{2}$${\mathrm{Zn}}_{3}$${(\mathrm{N}{\mathrm{O}}_{3})}_{12}$\ifmmode\cdot\else\textperiodcentered\fi{}24${\mathrm{H}}_{2}$O and ${\mathrm{La}}_{2}$${\mathrm{Mg}}_{3}$${(\mathrm{N}{\mathrm{O}}_{3})}_{12}$\ifmmode\cdot\else\textperiodcentered\fi{}24${\mathrm{H}}_{2}$O. The spectra of nearest-neighbor $X\ensuremath{-}Y$ and $X\ensuremath{-}X$ pairs have been identified and interpreted to obtain the single-ion properties of the members of the pairs and the spin-spin interactions between them. To an accuracy of 1%, the interaction between all pairs is described by the dipolar interaction plus an isotropic bilinear exchange $J{\stackrel{\ensuremath{\rightarrow}}{\mathrm{S}}}_{1}\ifmmode\cdot\else\textperiodcentered\fi{}{\stackrel{\ensuremath{\rightarrow}}{\mathrm{S}}}_{2}$. For nearest-neighbor $X\ensuremath{-}Y$ pairs, we find $J(X,Y)=0.096\ifmmode\pm\else\textpm\fi{}0.002$ ${\mathrm{cm}}^{\ensuremath{-}1}$ (antiferromagnetic), and for nearest-neighbor $X\ensuremath{-}X$ pairs, $J(X,X)=\ensuremath{-}0.095\ifmmode\pm\else\textpm\fi{}0.003$ ${\mathrm{cm}}^{\ensuremath{-}1}$ (ferromagnetic). These results apply to both the zinc and magnesium compounds. The experimental value for $J(X,Y)$ agrees encouragingly well with the value 0.100 calculated from a model for the exchange developed earlier for ${\mathrm{Co}}^{2+}$ ions in these two sites. The measured spin-spin interaction constants for the pairs are used to predict the paramagnetic absorption spectrum of ${\mathrm{La}}_{2}$${\mathrm{Ni}}_{3}$${(\mathrm{N}{\mathrm{O}}_{3})}_{12}$ \ifmmode\cdot\else\textperiodcentered\fi{} 24${\mathrm{H}}_{2}$O, which is found to agree with the experiment for only narrow ranges of zero-field splittings ($D$) for the $X$ and $Y$ ions. The spin-spin interactions and single-ion properties dictate an unambiguous choice for the ordered state of lanthanum-nickel double nitrate. It has proved possible to calculate the total ordering energy and the specific heat at temperatures well above and well below the ordering temperature 0.393 K. These detailed calculations are made possible by the fact that the zero-field splitting term for the $Y$ ion, $D(Y){[{S}_{z}(Y)]}^{2}$, is much larger than the spin-spin interactions, which leads to a strong suppression of the effects of the transverse part of the $X\ensuremath{-}Y$ interaction. We find that $D(Y)$ is near \char22{} 2.25 ${\mathrm{cm}}^{\ensuremath{-}1}$ at all temperatures and in all double nitrates that we have investigated. The results of these calculations are in good agreement with the experimental results obtained by other investigators. The results obtained for $J(X,X)$ provide additional, but not definitive, information on exchange mechanisms for the $X\ensuremath{-}X$ pairs.