Mixed network former effects in tellurite glass systems: Structure/property correlations in the system (Na2O)1/3[(2TeO2)x(B2O3)1 − x]2/3

Mixed-network former effects upon glass transition temperatures, densities and electrical conductivities have been studied in glasses in the system (Na2O)1/3[(2TeO2)x(B2O3)1 − x]2/3. Mixing of network formers B2O3 and TeO2 has a negative effect on ionic conductivities. Possible structural origins of this behavior have been studied by 11B, 23Na, and 125Te high-resolution and dipolar solid state nuclear magnetic resonance (NMR) techniques, O-1s X-ray photoelectron spectroscopy and Raman spectroscopy. A quantitative structural model has been developed that is consistent with all of the experimental data, and provides a detailed description of network modification processes, interactions and connectivities. All the O-1s XP spectra can be satisfactorily fitted with two line shape components with the parameters of the binary end-member glasses, providing no evidence for the possible formation of B-O-Te linkages. In addition, none of the 125Te and 11B NMR data shows significant proof of such linkages, even though their formation cannot be explicitly excluded. 23Na NMR data indicate a monotonic chemical shift trend consistent with proportional sharing of the network modifier between the two network former species. The experimental data suggest a tendency towards an avoidance of B-O-Te linkages, even though no macroscopic phase separation is observed. Nevertheless, the negative mixed network former effect can be rationalized in terms of compositionally segregated sodium borate and sodium tellurite nano-domains.