Electronic Spectral Study of Co(ll) Complexes in Potassium Borate Glasses with Low Alkali Oxide Contents

The density, refractive index, thermal expansion coefficient, and transformation range viscosity of binary GeO 2 -SiO 2 glasses have been measured. Results indicate that the density and refractive index of these glasses are additive as a function of the molar composition of the glass. The thermal expansion coefficient of these glasses exhibits a slightly positive deviation from additivity. The transformation range viscosity, which is reported here for the first time for binary GeO 2 -SiO 2 glasses, exhibits a strong negative deviation from additivity. These results can be explained by assuming that the structure of these glasses consists of a mixture, which may be either random or contain clusters, of Ge-O and Si-O tetrahedra and that viscous flow is controlled by breaking of Ge-O bonds. The compositional dependency of the viscosity is contrary to that expected for phase separated glasses.