Sodium germanate glasses and crystals: NMR constraints on variation in structure with composition

We present new results from high-resolution ¹⁷O and ²³Na NMR spectroscopy on sodium germanate glasses ranging from 4 to 36 mol% Na₂O, and on crystalline sodium digermanate (Na₂Ge₂O₅). Combined with previously published results, these provide a more complete, direct view of changes in oxygen speciation with composition, and the corresponding changes in Ge coordination. Until about 15–20% Na₂O, non-bridging oxygens are not detected (<2% of oxygens). At higher Na contents, NBO begin to form in significant quantities and, by 36% Na₂O, are at the level expected for a ‘silicate-like’ speciation reaction, with all four-coordinated Ge (^4]Ge). At intermediate compositions, a clear NMR signal is seen for oxygens bridging between one ^4]Ge and one ^5]Ge or ^6]Ge. Estimated mean Ge coordination numbers are consistent with previous X-ray and neutron scattering results. We observe systematic, monotonic changes in the NMR parameters for both Na⁺ cations and ^4]Ge–O–^4]Ge bridging oxygens, suggesting a lack of clustering. This behavior is analogous to that of alkali borate glasses, where network cation coordination is well-known to shift with composition, but is quite distinct from that of alkali silicates, where coordination changes are large only at high pressures.