Atomic site preferences and structural evolution in vanadium-dopedZrSiO4from multinuclear solid-state NMR

Abstract

Solid state NMR spectra of ${}^{29}\mathrm{Si}$ are reported from pure and vanadium-doped zircon $({\mathrm{V}\ensuremath{-}\mathrm{Z}\mathrm{r}\mathrm{S}\mathrm{i}\mathrm{O}}_{4})$ samples. The vanadium concentration is varied up to $\ensuremath{\sim}1\ensuremath{-}\mathrm{mol}%{\mathrm{V}}^{4+}$ by using both conventional-firing and sol-gel routes, and ${}^{51}\mathrm{V}$ NMR data are also recorded. ${}^{17}\mathrm{O}$ NMR of ${}^{17}\mathrm{O}$ isotopically enriched samples shows that the initial gel is completely amorphous with the whole range of possible $M\ensuremath{-}\mathrm{O}{\ensuremath{-}M}^{\ensuremath{'}}$ linkages detected, and that this structure evolves into a fully ordered ${\mathrm{ZrSiO}}_{4}$ structure with calcination. Static ${}^{91}\mathrm{Zr}$ NMR data is reported from a pure zircon sample. The NMR data are used to quantify the amount of vanadium entering the zircon structure, and to elucidate its site preference within the lattice. Two contact shifted peaks with very different ${T}_{1}$ relaxation from the main zircon peak but attributable to the zircon lattice are observed in the ${}^{29}\mathrm{Si}$ NMR spectra for all samples. These spectra are consistent with vanadium substitution on both the tetrahedral and dodecahedral sites, with a slight preference for the silicon site. The data show that the relative occupation of these two sites is almost independent of the preparation method and vanadium concentration. At a higher vanadium concentration a third additional peak is observed which may indicate another substitution site. Variable temperature NMR and susceptibility measurements indicate the hyperfine nature of the interactions influencing silicon from ${\mathrm{V}}^{4+}$ ions in the different sites.