Ionic transport in the 0.2[xNa2O▪(1-x)Rb2O]▪0.8B2O3 mixed-alkali system with x = 0, 0.2, 0.4, 0.6, 0.8 or 1.0 in the glassy and the undercooled-liquid state was investigated by means of impedance spectroscopy and tracer diffusion experiments. The calorimetric glass-transition temperature obtained by differential scanning calorimetry exhibited a minimum as a function of composition. The composition dependence of the electrical conductivity below Tg also exhibited a minimum. These deviations from a so-called ideal linear mixing rule were usually termed a mixed-alkali effect. The direct-current conductivities times temperature σdcT follow the Arrhenius law in the range below and above Tg, respectively. The glass transition appeared as a kink in the Arrhenius plot of σdcT. Below the glass-transition temperature the onset frequency, νon, of the conductivity dispersion had an Arrhenius-like temperature dependence. According to so-called Summerfield scaling the activation enthalpies of σdcT and νon were expected to be the same. This was indeed observed but only for the single-alkali compositions. The activation enthalpies of σdcT as a function of composition show a classical mixed-alkali maximum, however the activation enthalpies of the onset frequencies as a function of composition exhibit a nearly constant behavior in contrast to the expectation from Summerfield scaling. The tracer diffusion measurements reveal a major difference in diffusion of 86Rb and 22Na in mixed-alkali glasses. A diffusivity crossover of tracer diffusion coefficients of 22Na and 86Rb occurred near X = 0.2. By comparison of tracer and conductivity diffusivities the Haven ratio was deduced which exhibited a maximum near the conductivity minimum composition.

Ionic Conduction, Diffusion and Glass Transition in 0.2[xNa2O,(1-x)Rb2O]0.8B2O3. A.W.Imre, S.Voss, H.Mehrer: Journal of Non-Crystalline Solids, 2004, 333[3], 231-9