Germanate anomaly and its temperature dependence: An ultra-high field 17O NMR spectroscopic study of sodium germanate glasses

The oxygen speciation in (Na2O)x(GeO2)100-x glasses with 4 <= x <= 28 is studied using high-resolution 17O NMR spectroscopy at ultra-high field (35.2 T). The structure of glasses with x <= 18 consists of oxygen atoms bridging either two four-coordinated GeIV atoms (O44) or a GeIV and another six-coordinated GeVI atom (O46). The O46: O44 ratio monotonically increases with increasing Na2O content in this composition range. Further addition of Na2O results in a lowering of O46 and in a concomitant rise in the concentration of non-bridging oxygen (NBO) atoms. The resulting compositional variation in the average coordination number of Ge atoms is associated with the non-monotonic evolution of density and the germanate anomaly in these glasses. On the other hand, increasing fictive temperature results in a net conversion of GeVI -> GeIV + NBO in these glasses, which is argued to be a source of temperature dependent configurational entropy in germanate liquids.