Mixed-alkali effect in Na–Rb borate glasses: A tracer diffusion and electrical conductivity study

Abstract

The ionic transport in a series of 0.2 [X Na₂O·(1 − X)Rb₂O]·0.8B₂O₃ glasses has been studied by measuring both the electrical conductivity of these glasses and the tracer diffusivities of ²²Na and ⁸⁶Rb as functions of composition and temperature. At constant temperature (380 °C) the tracer diffusivities of ²²Na and ⁸⁶Rb decrease monotonically with decreasing Na- or Rb-content, respectively. The tracer diffusivities as functions of the composition parameter X intersect near X = 0.2 (‘crossover composition’). This behaviour gives rise to a minimum in the conductivity which appears near X = 0.4. At the crossover composition X = 0.2 the diffusivities of ²²Na and ⁸⁶Rb are practically the same within the measured temperature range resulting in similar Arrhenius parameters. For each other investigated composition the Arrhenius parameters for ²²Na and ⁸⁶Rb diffusion differ significantly. Comparing the ²²Na-diffusivity in the pure Na-borate glass with the corresponding conductivity diffusion coefficient calculated via the Nernst–Einstein equation from the measured conductivity yields a temperature-independent Haven ratio. In contrast, a temperature-dependent Haven ratio was found in the pure Rb-borate glass. In order to compare the tracer diffusivities in mixed-alkali glasses with their ionic conductivity we introduce a ‘common Haven ratio’ for the mixed-alkali glasses which coincides with the conventional definition of the Haven ratio for single-alkali glasses. On the Rb-rich side the common Haven ratio decreases with temperature for Na–Rb borate glasses (X = 0.2 and X = 0.4) similar to the Haven ratio in the pure Rb borate glass.