Does six-coordinate germanium exist in Na 2O–GeO 2 glasses? Oxygen-17 nuclear magnetic resonance measurements

Abstract

Sodium germanate glasses of compositions Na 2O · 9GeO 2 and 2Na 2O · 9GeO 2 have been studied using solid-state 17O nuclear magnetic resonance (NMR). Quartz-type and rutile-type GeO 2 were used as examples of oxygen connected to 4-fold and 6-fold coordinated germanium. The 17O spectra yielded nuclear quadrupole coupling constants, ( e 2 qQ/ h), asymmetry parameters, ( η), and isotropic chemical shifts, ( δ i ), from line shape simulations. The chemical shifts of oxygen in [GeO 4] and [GeO 6] units in germania are 70 ± 5 and 160 ± 5 ppm, respectively, with only a small difference in the quadrupole coupling constant but a larger difference in the asymmetry parameter. In the Na 2O–GeO 2 glasses, the quadrupole coupling constant decreases with addition of Na 2O to the glass with no evidence for any oxygen with a smaller electric field gradient, indicating that there are very few non-bridging oxygens (NBOs) present. The 17O chemical shift in Na 2O · 9GeO 2 is similar to that in [GeO 6] while in 2Na 2O · 9GeO 2 the chemical shift is close to that of [GeO 4].