Ionic conduction, diffusion and glass transition in 0.2[XNa2O · (1−X)Rb2O] · 0.8B2O3

We have investigated the ionic transport in the 0.2XNa₂O · (1−X)Rb₂O] · 0.8B₂O₃ mixed-alkali system with X=0.0, 0.2, 0.4, 0.6, 0.8, and 1.0 in the glassy and the undercooled-liquid state by means of impedance spectroscopy and tracer diffusion experiments. The calorimetric glass-transition temperature T_g obtained by differential scanning calorimetry shows a minimum with composition. The composition dependence of the electrical conductivity below T_g exhibits a minimum, as well. These deviations from an ‘ideal’ linear mixing rule are usually denoted as mixed-alkali effect. The dc conductivities times temperature σ_dc×T follow the Arrhenius law in the range below and above T_g, respectively. The glass transition appears as a kink in the Arrhenius presentation of σ_dc×T. Below the glass-transition temperature the onset frequency ν_on of the conductivity dispersion has an Arrhenius-like temperature dependence. According to ‘Summerfield scaling’ the activation enthalpies of σ_dc×T and ν_on are expected to be the same. This is indeed observed but only for the single-alkali compositions. The activation enthalpies of σ_dc×T 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 occurs near X=0.2. By comparison of tracer and conductivity diffusivities the Haven ratio is deduced which shows a maximum near the conductivity minimum composition.