Ionic transport in 0.2[XNa2O·(1–X)Rb2O]·0.8B2O3 mixed-alkali glasses

Mixed sodium–rubidium borate glasses with compositions 0.2[XNa2O·(1-X)Rb2O]·0.8B2O3 (X = 1.0; 0.8; 0.6; 0.4; 0.2; 0.0) were prepared from alkali-metal carbonates and diboron trioxide. The density ρ and the molar volume Vmol of the prepared glasses vary almost linearly with composition whereas the glass-transition temperature Tg shows a minimum between X = 0.2 and X = 0.4. Measurements of the electrical conductivity using an ac complex impedance technique have been performed between 5 Hz and 1.3 MHz over the entire composition range. The conductivity diffusion coefficient, Dσ, of the mobile alkali-metal ions was deduced from the electrical dc conductivity via the Nernst–Einstein relation. The diffusivity has a minimum and the activation enthalpy a maximum at X ≈ 0.4 indicating the mixed-alkali effect. Supposing an Arrhenius-type temperature dependence the activation parameters and pre-exponential factors were determined. The deviations from the Arrhenius function were also studied and found to be most pronounced around X = 0.2 to 0.4. The time-dependent isotherm of the diffusivity of 0.2Na2O·0.8B2O3 glass was investigated at about 40 K below Tg and attributed to the relaxation of the glass network.