Infra-red detection of depolymerization in Tl2O-GeO2-SiO2 glasses

Abstract

Three series of glasses containing 15, 26, and 35 mol % Tl2O were prepared in the Tl2O-GeO2-SiO2 system and their infra-red spectra, densities, and refractive indices determined. Refraction decreases as SiO2 substitutes for GeO2 and is additive with respect to the two binary series, except for slight positive deviations with the low SiO2content 35 mol % Tl2O glasses. All glass series exhibit small negative molar volume deviations from additivity at low SiO2 contents and small positive deviations from additivity at higher SiO2 contents. The strong compositional dependence of the main infra-red stretching frequency values (1100 cm−1 for SiO2 and 875 cm−1for GeO2) are shown to be relatively sensitive indicators of changes in the polymeric makeup of these ternary oxide glasses. Loosening and/or depolymerization of either pure or damaged MO4 networks is accompanied byv Si-O and/orv Ge-O shifts to smaller frequencies. The frequency changes that accompany the progressive replacement of GeO2 by SiO2 suggest that (a) SiO4 tetrahedra initially replace GeO4 tetrahedra, (b) GeO6 octahedra subsequently transform to GeO4 tetrahedra, and (c) GeO4 tetrahedra finally take part in repair of damaged SiO4 networks. Use of a previously developed infra-red isofrequency contour technique revealed similar ternary trends for the two vibrational modes of interest. This emphasizes the ability of such contour trends to reflect the overall degree of network crosslinking present in ternary oxide glasses.