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 similar to17.5-20 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 Ge-IV to Ge-VI 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 1 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 Q(3) non-bridging oxygens (NBOs) and at higher alkali oxide contents, Q(2) 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 small-ring 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.