Network former mixing (NFM) effects in alkali germanotellurite glasses

Network former mixing (NFM) effects on ionic conductivities have been characterized in ternary germanotellurite glasses of composition (A2O)0.3[(TeO2)x(GeO2)1-x]0.7 (A = Li, Na), revealing an interesting cation dependence. While the results demonstrate a weak positive NFM effect in the Li-containing system, the electrical conductivities of the Na-containing systems correspond to the weighted average values observed in their binary endmembers. The structural origins of these differing effects have been explored by Raman scattering and 125Te solid-state NMR spectroscopy, indicating no significant differences in the structural organization of these materials. Overall, the data suggest that the concentration of heteroatomic connectivities is relatively low. For the Li-containing system, an analysis of the motional narrowing effects observed in temperature-dependent static 7Li NMR spectra confirms the existence of a weak positive NFM effect at the local atomic level. Accordingly, the absence of an NFM effect for the Na-containing system is corroborated by low-temperature 23Na spin-echo decay spectroscopy, showing very similar network modifier distributions in those glasses.