The structure of glasses along the Na 2OGeO 2 join

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Raman spectra of glasses with compositions along the Na 2OGeO 2 join and corresponding crystalline phases have been investigated using micro-Raman techniques. The main vibrational band in the Raman spectrum of GeO 2, which is associated with symmetric stretch of the GeOGe bonds, is observed to contain fine structure. These weak vibrational features may be indicative of several distinct 4-membered ring populations within the network. Changes in the intensity of the fine structure with addition of Na 2O indicate that the ring statistics change systematically with composition. No evidence is observed along the join for the formation of 6-fold coordinated germanium atoms. The ‘germanate anomaly’ exhibited by these glasses appears to result from the formation of 3-membered rings of GeO 4 tetrahedra. The maximum (or minimum) in the anomaly occurs when the network becomes saturated in 3-membered GeO 4 rings. Beyond this point, the continued formation of Q 3 tetrahedra (tetrahedra containing 1 NBO) is responsible for the decline (increase) in some properties. The glass structure begins to resemble the crystalline digermanate structure at ≥ 30–35 mol% added Na 2O. Crystallization occurs at ∼ 40 mol% Na 2O compositions at which large numbers of Q 2 tetrahedra form and there is total breakdown of the glass network.