V51NMR studies of transferred hyperfine effects in the rare-earth vanadates

${}_{}{}^{51}{}_{}{}^{}\mathrm{V}$ ($I=\frac{7}{2}$) NMR investigation in the whole set of rare-earth vanadates ($R\mathrm{V}{\mathrm{O}}_4$) in the polycrystalline form at 300 K has revealed transferred hyperfine effects. It has been observed that the ${}_{}{}^{51}{}_{}{}^{}\mathrm{V}$ NMR line shape strongly depends on the relative strength of the magnetic and quadrupolar parts of the transferred hyperfine interaction. On the basis of the powder pattern the $R\mathrm{V}{\mathrm{O}}_4$ system may be divided into two halves. The first-half members always exhibit a quadrupolar split central transition in the frequency range 2-12 MHz, whereas the members of the second half exhibit at higher frequencies an asymmetric line which, however, splits as the frequency is lowered. The hyperfine-interaction parameters, viz., the isotropic shift $K_{\mathrm{iso}}$, the anisotropic shift $K_{\mathrm{an}}$, and the quadrupolar coupling constant $\frac{e^2\mathrm{qQ}}{h}$ have been estimated from the central transition. The linear plot of $K_{\mathrm{iso}}$ against calculated $〈S_z〉$ values of rare-earth ions is indicative of the contact nature of $K_{\mathrm{iso}}$. Though $K_{\mathrm{an}}$ is dominated by the dipolar part, a small amount of anisotropic hyperfine part $K_{\mathrm{an}}^{\mathrm{hf}}$ could be separated from the experimental shift. Thus the results of both $K_{\mathrm{iso}}$ and $K_{\mathrm{an}}^{\mathrm{hf}}$ seem to uphold the presence of a small amount of covalent contribution ($f_s\sim 0.01\%$ and $f_s\sim 0.0001\%$) in these basically ionic compounds. A comparison of the ligand NMR shifts in $3d$ and $4f$ systems (oxysalts) suggests that the mechanism of spin transfer within the oxyanion is not the same in these two series. Finally, the $\frac{e^2\mathrm{qQ}}{h}$ values (0.400-0.300 MHz) exhibit a gradual decrease along the isostructural series of $R\mathrm{V}{\mathrm{O}}_4$. The value of the coupling constant and its variation across the rare-earth series in different compounds have been qualitatively interpreted by taking into account the bonding effects.