Mixed-Alkali Effect of Tracer Diffusion and Ionic Conduction in Na-Rb Borate Glasses as a Function of Total Alkali Content

The mixed-alkali effect is studied in Y [X Na2O · (1−X) Rb2O] · (1−Y) B2O3 glasses as a function of the total alkali content Y. To this aim the ionic transport in glasses with Y = 0.3 was investigated by impedance spectroscopy and by the radiotracer diffusion technique. The results are compared with data obtained earlier by our group on a glass system with Y = 0.2. In both systems the relative compositions X = Na/(Na + Rb) are varied from 0.0 to 1.0 in steps of 0.2. The dc electrical conductivity is higher in Y = 0.3 glasses than in Y = 0.2 glasses and exhibits a minimum in each system near X = 0.4. Also near X = 0.4 a pertaining maximum in activation enthalpy occurs being more pronounced for Y = 0.3 as compared to Y = 0.2. The glass-transition temperature T g is almost identical for glasses with Y = 0.2 and 0.3 with a minimum near X = 0.2. The diffusivities of 22Na and 86Rb measured at a constant temperature (380 °C) show in both glass systems (Y = 0.3 and Y = 0.2) a similar dependence on the composition parameter X. The 86Rb diffusivity decreases exponentially with decreasing Rb content. The 22Na diffusivity decreases with decreasing Na content but reaches an almost constant plateau on the Rb-rich side. For each relative composition X the diffusivities of the two isotopes increase by nearly the same constant factor, when Y is increased from 0.2 to 0.3. In both systems 22Na- and 86Rb-diffusivities as functions of the composition parameter X intersect near X = 0.2 explaining the minimum in dc-conductivity. Composition dependent common Haven ratios were obtained by comparing the conductivity with the diffusion data.