Structural impact of niobium oxide on lithium silicate glasses: Results from advanced interaction-selective solid-state nuclear magnetic resonance and Raman spectroscopy

Abstract

While niobium-containing oxide glasses are used in many technological applications, especially optical glasses, the exact structural role of Nb and its impact on the structure of covalent/ionic oxide glasses remain incompletely understood. In the present study, this issue is addressed for Nb2O5-containing lithium silicate glasses using 7Li, 29Si, and 93Nb magic-angle-spinning (MAS) and advanced NMR methods sensitive to 7Li–7Li and 7Li–93Nb magnetic dipole-dipole interactions, accompanied by molecular dynamics (MD) and Monte-Carlo (MC) simulations. In glasses along the compositional line (1–y)Li2Si2O5–yNb2O5, 29Si MAS-NMR spectra reveal a gradual silica network repolymerization with increasing y, indicating that the accommodation of the niobium oxide component into the glassy network requires on average about 0.7 equivalents of lithium oxide. Significant 7Li–93Nb interactions were also detected and quantified by Rotational Echo Saturation Pulse Double Resonance (RESPDOR) experiments. Consistent with this finding, profound changes in the local environments of the lithium ions and their spatial distributions are observed in 7Li satellite transition (SATRAS) spectra and 7Li dipolar spin echo decays, whereas 93Nb NMR and Raman spectra suggest the formation of six-coordinated NbO6 polyhedra with both, corner- and edge-sharing connectivity. Results obtained on glasses along a second composition line, SiO2–[(Li2O)1.4(Nb2O5)], confirm the above-described structural concepts. The average homo- and heteronuclear dipolar interaction strengths as expressed by the second moments M2(Li-Li) and M2(Li-Nb) are consistent with random spatial distributions of the lithium and niobium species, with a tendency towards Nb clustering at higher concentrations. All of these results suggest that niobium oxide adopts a network former role in these glasses. The current study defines a comprehensive general strategy for elucidating the structural impact of NMR active intermediate oxides incorporated into silicate glasses.