THE STRUCTURE OF ALKALINE-EARTH ALUMINOSILICATE GLASSES AS DETERMINED BY VIBRATIONAL SPECTROSCOPY

The structure of glasses in the CaO-Al2O3-SiO2 (CAS) and MgO-Al2O3-SiO2 (MAS) systems have been studied using Raman and infrared (IR) reflectance spectroscopies. Kramers-Kronig analysis was performed on the IR data. Two compositional series were chosen in each system, one which has a constant average polymerization of 1 non-bridging oxygen per tetrahedral cation, and the other in which silica content is constant at 50 mol%, but polymerization varies. The results indicate that the presence of alkaline earth network modifiers favors the formation of species with high negative charge density. In the NBO/T = 1 series this leads to the equilbrium 2Q3 = Q4 + Q2 (where the superscript is the number of bridging oxygens per tetrahedron) shifting to the right at low Al content and to the left at high Al/Si. In the fully polymerized, 50% silica Mg glass, there is evidence of clustering into Al-rich and Si-rich domains. Transformation of the IR data yields a well-resolved low-frequency band in the CAS spectra whose frequency depends on polymerization and whether Ca is acting as a network modifier or charge balancer. The analogous Mg band is apparently buried in the mid-frequency network mode.