Mixed Network Former Effects in Tellurite Glass Systems: Structure/Property Correlations in the System (Na₂O)_1/3[(2TeO₂)_<i>x</i>(P₂O₅)_1–<i>x</i>]_2/3

Abstract

Mixed network former glasses in the system (Na 2 O) 1/3 [(2TeO 2 ) x (P 2 O 5 ) 1– x ] 2/3 have been prepared and characterized by DSC, density, and electrical conductivity measurements. Within the compositional region 0 < x < 0.5, the electrical conductivity shows a positive mixed network former effect, i.e., enhanced glass transition temperatures and ionic conductivities in relation to the linearly interpolated values between the corresponding binary sodium tellurite and sodium phosphate systems. The structural origins of this effect have been studied by 31 P, 23 Na, and 125 Te high-resolution and dipolar solid-state nuclear magnetic resonance (NMR) techniques, O-1s, Na-1s, Te-3d 5/2, and P-2p X-ray photoelectron spectroscopy (XPS) 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. For x values <0.5, the data evidence a preferential formation of Te–O–P linkages, whereas at higher x values, the connectivity distribution appears to be more random. 23 Na NMR data suggest a dominant association of the network modifier species with the phosphate species for x ≤ 0.5, whereas for larger x values the participation of nonbridging oxygen atoms associated with tellurium becomes increasingly evident in 23 Na{ 31 P} rotational echo double resonance (REDOR) results. The enhanced T g values and electrical conductivities can be directly correlated with the preferential formation of Te 3 4 –O–P (3) and Te 1 3 –O–P (3) linkages, which both strengthen the network and create shallower Coulomb potentials via anionic charge delocalization.