Structure of SiO2–Al2O3 glasses: Combined X-ray diffraction, IR and Raman studies

The average structures of roller-quenched (SiO2)1−x(Al2O3)x, x = 0.25, 0.38, 0.53 and 0.60, glasses have been analyzed by large-angle X-ray scattering, IR and Raman spectroscopies. The changes of the local structure around the Si and Al atoms with composition are investigated. The average T–O distances for the glasses with 25 and 38 mol% Al2O3 are very similar to the average tetrahedral T–O distances calculated from ionic radii. The estimated coordination numbers of these glasses are about 4. On the other hand, the average T–O distances and coordination numbers of the glasses with 53 and 60 mol% Al2O3 suggest the existence of AlO5 and AlO6 polyhedra. However, models of the first T–O peak in the radial distribution functions and the IR spectra indicate that the actual number of AlO5 polyhedra should be very small in these glasses. The polarized Raman spectra indicate that a homogeneous SiO2–Al2O3 network is formed in which the relative number of interconnected SiO4 tetrahedra gradually decreases with increasing Al2O3 content. Comparison with SiO2–Al2O3 liquid structures, previously derived from molecular dynamics simulations, suggest that the AlO5 species, which are dominant in the high-temperature liquids, transform to AlO4 and AlO6 species despite the relatively rapid quenching method used.