Electron Paramagnetic Resonance ofGd3+Pairs in Rare-Earth Hydroxides

Electron paramagnetic resonance has been observed from isolated nearest-neighbor (nn) and next-nearest-neighbor (nnn) pairs of ${\mathrm{Gd}}^{3+}$ in Y${\mathrm{}\left(\mathrm{OH}\right)\mathrm{}}_3$ and Eu${\mathrm{}\left(\mathrm{OH}\right)\mathrm{}}_3$ at 77 K and 23 GHz. The spectra were analyzed in the same way as previous measurements on ${\mathrm{Gd}}^{3+}$ pairs in La${\mathrm{Cl}}_3$ and Eu${\mathrm{Cl}}_3$ and, as before, the dominant interactions were found to be isotropic bilinear exchange ($J{\mathrm{S}}_1·{\mathrm{S}}_2$) and magnetic dipole coupling, with small crystal-field terms. However, the relative strengths of the interactions turned out to be rather different, and in the case of the nearest neighbors this gave rise to some interesting complications. It was found that all of the observable lines (about 40 in each case) were almost equally sensitive to bilinear and biquadratic isotropic interactions, with the result that only the combination $J_{}^{\prime }{}_{\mathrm{nn}}{}^{}=J_{\mathrm{nn}}-41\mathrm{}{Q}_{\mathrm{nn}}$ could be determined accurately, where $Q_{\mathrm{nn}}$ is the coefficient of the biquadratic coupling expressed in the form $Q_{\mathrm{nn}}\Sigma {m=2\mathrm{}}^{m=-2\mathrm{}}{\mathrm{}(-1\mathrm{})\mathrm{}}^m{O}_m^{\mathrm{}\left(2\right)\mathrm{}}\left(S_1\right)O_{-m}^{\mathrm{}\left(2\right)\mathrm{}}\left(S_2\right)$. For the next-nearest-neighbor pairs no such complication was evident and a unique fit was obtained with $Q_{\mathrm{nnn}}=0\mathrm{}$. $J_{}^{\prime }{}_{\mathrm{nn}}{}^{}$ and $J_{\mathrm{nnn}}$ were fitted to the observed spectra and the following values were obtained: $J_{}^{\prime }{}_{\mathrm{nn}}{}^{}\left[\mathrm{Y}{\left(\mathrm{OH}\right)}_3\right]=0.164\mathrm{}\pm 0.006$ ${\mathrm{cm}}^{-1\mathrm{}}$, $J_{\mathrm{nnn}}\left[\mathrm{Y}{\left(\mathrm{OH}\right)}_3\right]=-0.0060\mathrm{}\pm 0.0001$ ${\mathrm{cm}}^{-1\mathrm{}}$; and