Glasses with a constant total content (30mol%) of alkali oxides were studied, and the diffusion of 22Na and 86Rb was determined at 673K by using radio-tracer and serial sectioning methods. It was found that the diffusion coefficients of Na and Rb exhibited a concentration dependence which disagreed with classical predictions for mixed-alkali glasses. No crossover of the diffusivities was observed, since the Na diffusion was always faster than Rb diffusion. The Na diffusivity went through a minimum at near to 60%Na2O, while the Rb diffusivity exhibited a maximum near to 50%Na2O. The ionic conductivity, as deduced from these data, exhibited a minimum as a function of the relative composition of Na and Rb, as in classical mixed-alkali glasses.

Tracer Diffusion in Sodium-Rubidium Borate Glasses - an Unconventional Mixed-Alkali Effect? U.Schoo, C.Cramer, H.Mehrer: Solid State Ionics, 2000, 138[1-2], 109-14