Q speciation in alkali germanate glasses

We have curve fit a series of alkali germanate glasses (R2O–GeO2, R = Na, K, Rb and Cs). Fits of the high-frequency (700–1100 cm− 1) region of the Raman spectra indicate that NBO vibrations are present, albeit very weak, in all compositions, including those below the germanate anomaly maxima. In addition Q2 species appear to be in greater abundance than Q3 in this composition range. Above anomaly compositions, the dominant Q species is Q3 followed by Q2 species being observed at 35–40 mol% alkali oxide. The Q3/Q2 intensity ratio is alkali dependent; the larger alkalis have fewer Q2 species present. A band at ∼ 746 cm− 1 is indicative of a higher coordinated Ge species. It occurs at ∼ 10 mol% alkali and is observed at low intensity in the highest alkali-containing glasses. For the Na2O–GeO2 glasses its intensity reaches a maximum at 20 mol% Na2O. Similar maxima are observed in Rb2O- and Cs2O-containing glasses also at the 20 mol% maximum. K2O-containing glasses have a maximum at 15 mol% alkali. The intensity of the band is alkali dependent suggesting that the concentration of higher coordinated Ge species decreases in the sequence Rb2O ≅ Cs2O < K2O < Na2O, consistent with recent neutron and X-ray diffraction studies.