Raman spectra and structure of sodium aluminogermanate glasses

Abstract

Polarized Raman spectra of x NaAlO 2·(100 − x) GeO 2 glasses ( x = 0, 5, 10, 15, 20, 25, 33, 42, and 50) are presented. Analyses of the Raman data indicate that the aluminogermanate glasses have three-dimensional network structures consisting of interconnected AlO 4 and GeO 4 tetrahedra; Na + ions are present in cavities and charge balance the Al 3+ ions. Systematic changes are observed in the frequencies, intensities and polarization characteristics of spectral bands with variations in the NaAlO 2 content of these glasses. The antisymmetric stretching mode [ν as (TOT), where T = Al, Ge] in the high-frequency region of the spectra (800–1000 cm −1) appears as a doublet consisting of well-defined bands in the spectra of glasses along the entire join. Both components of the high-frequency doublet shift to a lower frequency with increasing NaAlO 2 content, indicating that the ν as (GeO 4) and ν as(AlO 4) stretching modes are coupled. The variations in the TO force constants and TOT bond angles with change in composition most likely cause the bands to shift. The frequencies of the Raman bands of sodium aluminogermanate glasses are compared with those of the corresponding bands in isostructural sodium gallogermanate glasses. On the basis of this comparison, the origin and delocalization of the vibrational modes producing characteristic Raman bands in the spectra of these glasses are discussed. The changes observed in the Raman spectra of aluminogermanate glasses with variation in NaAlO 2 content are analogous to those observed in the spectra of glasses along the NaAlO 2SiO 2 join.