P31NMR studies of transferred hyperfine effects in rare-earth orthophosphates

Systematic ${}_{}{}^{31}{}_{}{}^{}\mathrm{P}$ NMR studies at 300 K of the 13 rare-earth phosphates ($R\mathrm{P}{\mathrm{O}}_4$) in the polycrystalline form have revealed the presence of transferred hyperfine effects. Two types of line shapes are broadly encountered: more or less symmetric lines from La to Eu, and asymmetric lines from Gd to Yb, determined by the relative magnitude of the anisotropic interaction and the intrinsic linewidth. The linewidths are in accord with the $R^{3+}$ magnetic moments except for ${\mathrm{Gd}}^{3+}$, the $S$-state ion. The powder pattern yields the hyperfine-interaction parameters, viz., the isotropic shift $K_{\mathrm{iso}}$ for all the members and the anisotropic shift $K_{\mathrm{aniso}}$ for Gd to Yb only. $K_{\mathrm{iso}}$ for the first half of the members is an order of magnitude smaller than for the second half of the members and is primarily "contact" in origin, whereas $K_{\mathrm{aniso}}$ is determined by the dipolar contribution. Moreover, $K_{\mathrm{iso}}$ changes sign at ${\mathrm{Eu}}^{3+}$ and not at ${\mathrm{Gd}}^{3+}$ in consonance with the $〈S_z〉$ values calculated by Golding and Halton, and so their model appears to be applicable to the second-nearest neighbor. The transferred hyperfine effects do seem to indicate a small degree of covalent interaction in these predominantly ionic compounds ($f_s$ varying from 0.001% to 0.015%), but the shift does not appear to support $f$-electron participation in the covalent bond. Finally, it has been shown that the sign of the shift of the second-nearest neighbor (${}_{}{}^{31}{}_{}{}^{}\mathrm{P}$ in $R\mathrm{P}{\mathrm{O}}_4$) is opposite to that of the first neighbor and as such differs from those in $3d$ systems.