SI-29 NMR AND INFRARED REFLECTANCE SPECTROSCOPY OF LOW-SILICA CALCIUM ALUMINOSILICATE GLASSES

Selected calcium aluminosilicate glass compositions have been investigated using nuclear magnetic resonance (NMR) and infrared reflectance spectroscopy, with emphasis on compositions in the silica-poor portion of the system. In particular, a structural explanation for a maximum observed in the glass transition temperature at roughly 15 Mol% SiO2 was sought. Si-29 chemical shifts from magic-angle spinning NMR data are nearly constant in the low-silica glasses and are consistent with depolymerized Q2 species, or fully polymerized Q4 tetrahedra with 4 Al next-nearest neighbors. Static Si-29 NMR spectra bear no evidence of asymmetric Q2 or Q3 sites; however, the asymmetric component could be hidden by extreme broadening due to structural disorder. The functions epsilon" and -Imag(1/epislon) have been calculated from infrared reflectance spectra by a Kramers-Kronig transformation. A change in lineshape which parallels the maximum in T(g) is attributed to variation in the distribution of tetrahedral aluminate species.