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 Ge^IV atoms (O⁴⁴) or a Ge^IV and another six-coordinated Ge^VI 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 Ge^VI → Ge^IV + NBO in these glasses, which is argued to be a source of temperature dependent configurational entropy in germanate liquids.