Does six-coordinate germanium exist in Na2O-GeO2 glasses?: Oxygen-17 nuclear magnetic resonance measurements

Sodium germanate glasses of compositions Na2O . 9GeO(2) and 2Na(2)O . 9GeO(2) have been studied using solid-state O-17 nuclear magnetic resonance (NMR). Quartz-type and rutile-type GeO2 were used as examples of oxygen connected to 4-fold and 6-fold coordinated germanium. The O-17 spectra yielded nuclear quadrupole coupling constants, (e(2)qQ/h), asymmetry parameters, (eta) and isotropic chemical shifts, (delta(i)): from line shape simulations. The chemical shifts of oxygen in [GeO4] and [GeO6] 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 Na2O-GeO2 glasses, the quadrupole coupling constant decreases with addition of Na2O 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 O-17 chemical shift in Na2O . 9GeO(2) is similar to that in [GeO6] while in 2Na(2)O . 9GeO(2) the chemical shift is close to that of [GeO4]. (C) 1998 Elsevier Science B.V. All rights reserved.