Structures and optical properties of tellurite glasses and glass ceramics

The structures and optical properties of (K2O)15(Nb 2O5)15(TeO2)70 glass and glass ceramic have been studied in order to understand the second harmonic generation observed from the glass ceramic. We have used 93Nb NMR, Raman spectroscopy, differential scanning calorimetry, small angle x-ray scattering, transmission electron microscopy, and powder x-ray and neutron scattering. We find that there is a microstructure consistent with binodal phase separation leading to spherical inclusions ∼20 nm in size. Upon heat treatment, these domains become nanocrystals of K2Te 4O9. A theory of optical heterogeneity is used to describe the observed second harmonic generation which is ∼95 times more intense that quartz. The chi(2) value for this material is 3.0 x 10-9 esu.; A second project has used 125Te and 17O NMR to study alkali tellurite glasses in the system (M2O) x(TeO2)10-x, where M = Li, Na or K and x = 1, 2 or 3. The 125Te results show that complex models of network modification are needed to explain the resulting spectra that include a distribution of polyhedral tellurite units at all compositions. The 17O results show that there is a clear distinction between bridging and non-bridging oxygen sites in tellurite crystals and that sophisticated NMR experiments should be able to distinguish them in the glasses. Further, we have used Extended Huckel theory tight-binding calculations to predict the 17O NMR shifts of SiO2, GeO 2 and TeO2. We find that these calculations allow accurate predictions of the chemical shifts based solely on the trend in valence orbital size, and that expensive calculations of electron currents need not be used for this application.