Q speciation in alkali germanate glasses

Abstract

We have curve fit a series of alkali germanate glasses (R 2O–GeO 2, 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 Q 2 species appear to be in greater abundance than Q 3 in this composition range. Above anomaly compositions, the dominant Q species is Q 3 followed by Q 2 species being observed at 35–40 mol% alkali oxide. The Q 3/Q 2 intensity ratio is alkali dependent; the larger alkalis have fewer Q 2 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 Na 2O–GeO 2 glasses its intensity reaches a maximum at 20 mol% Na 2O. Similar maxima are observed in Rb 2O- and Cs 2O-containing glasses also at the 20 mol% maximum. K 2O-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 Rb 2O ≅ Cs 2O < K 2O < Na 2O, consistent with recent neutron and X-ray diffraction studies.