Germanium coordination and the germanate anomaly

The structural mechanism responsible for the germanate anomaly along the X₂O-GeO₂ join, where X = Li, Na, K, Rb, and Cs, was investigated using Raman spectroscopy. Density maxima were measured at ∼17.5–20 mol%, 15 mol%, and 10 mol% for Li₂O, Na₂O and K₂O, respectively. A maximum at 15 mol% and a minimum at 32.5 mol% were measured for Rb₂O while the Cs₂O glasses exhibited a maximum at 17.5 mol%. A coordination change of ^IVGe to ^VIGe was not observed. It is proposed that the germanate anomaly is a consequence of formation of small 3-membered GeO₄ rings as alkali oxide is added to the glass. The small rings cause a density increase. The anomaly maximum is reached when continued small ring formation cannot occur without straining the glass network. At this point the network generates large numbers of Q³ non-bridging oxygens (NBOs) and at higher alkali oxide contents, Q² NBOs. The formation of the NBOs causes a decline in the density. The Rb₂O- and Cs₂O-containing glasses do not exhibit similar density trends to the lighter alkali-containing glasses. This is because of competing density effects from smallring formation and the increased mass of the alkalis. The overall effect on the density trends of these glasses is to skew the anomaly maxima to higher alkali oxide compositions.