The Germanate Anomaly: What do we know?

The structural mechanism responsible for the germanate anomaly remains an area of interest to the glass sciences. The currently accepted model for the mechanism is a coordination change of Ge from 4-fold to a higher coordinated species. Recent work has shown that the higher coordinated species is 5-fold Ge. However, while the formation of the higher coordinated species clearly plays a role in the anomalous behavior it does not correlate with the anomaly maximum. Raman spectroscopy has shown that Raman bands attributed to small ring vibrations exhibit a maximum in intensity at the anomaly maximum and, in addition, show that the higher coordinated Ge species occurs at the onset of Q3 formation. The structural mechanism responsible for the anomaly remains inconclusive but appears to involve a complex interaction between the formation of 5-fold Ge, generation of Q3 and Q2 species, and formation of small 3-membered GeO4 rings. Further work, particularly high resolution neutron scattering on low alkali compositions before the anomaly, as well as high alkali compositions beyond 33 mol%, are needed to resolve many of the inconsistencies between different experimental results.