Pair Spectra and Magnetic Properties ofNi2+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 ${\mathrm{La}}_2$ ${\mathrm{Zn}}_3$ ${\left(\mathrm{N}{\mathrm{O}}_3\right)}_{12}$·24${\mathrm{H}}_2$O and ${\mathrm{La}}_2$ ${\mathrm{Mg}}_3$ ${\left(\mathrm{N}{\mathrm{O}}_3\right)}_{12}$·24${\mathrm{H}}_2$O. The spectra of nearest-neighbor $X-Y$ and $X-X$ 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 $J{\overrightarrow{\mathrm{S}}}_1·{\overrightarrow{\mathrm{S}}}_2$. For nearest-neighbor $X-Y$ pairs, we find $J(X,Y)=0.096\mathrm{}\pm 0.002$ ${\mathrm{cm}}^{-1\mathrm{}}$ (antiferromagnetic), and for nearest-neighbor $X-X$ pairs, $J(X,X)=-0.095\mathrm{}\pm 0.003$ ${\mathrm{cm}}^{-1\mathrm{}}$ (ferromagnetic). These results apply to both the zinc and magnesium compounds. The experimental value for $J(X,Y)\mathrm{}$ 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 ${\mathrm{La}}_2$ ${\mathrm{Ni}}_3$ ${\left(\mathrm{N}{\mathrm{O}}_3\right)}_{12}$ · 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...