Thermal and electrical properties of mixed alkali in Li2O-Na2O-WO3-P2O5 glasses

A series of tungsten-phosphate glasses, 0.5[xNa(2)O-(1-x)Li2O] - 0.5[0.25(WO3)(2) - 0.75P(2)O(5)] with x varying between 0 and 1, were prepared using the melt quenching technique. They were studied by using powder X-ray diffraction (XRD), differential thermal analysis (DTA) and impedance spectroscopy. The total alkali oxide content is chosen in such a way that the electrical conductivity of the glasses under study is entirely ionic. It was evidenced that the conductivity minimum corresponds to the maximum of the activation energy. The glass transition temperature presents a minimum for the ratio Na/(Na + Li) = 0.5. On the other hand, the density varies linearly versus Na/(Na + Li). The mixed-alkali effect (MAE) is resulting here in a conductivity minimum observed for the composition corresponding to Na/(Na + Li) - 0.5, (x = 0.5). It could be attributed to the fact that the two types of alkali ions are randomly mixed and have distinct conduction pathways. (C) 2008 Elsevier B.V. All rights reserved.