Germanium coordination and the germanate anomaly

The structural mechanism responsible for the germanate anomaly along the X 2 O-GeO 2 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 Li 2 O, Na 2 O and K 2 O, respectively. A maximum at 15 mol% and a minimum at 32.5 mol% were measured for Rb 2 O while the Cs 2 O glasses exhibited a maximum at 17.5 mol%. A coordination change of IV Ge to VI Ge was not observed. It is proposed that the germanate anomaly is a consequence of formation of small 3-membered GeO 4 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 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 Rb 2 O- and Cs 2 O-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.