Tracer Diffusion of ~(22)Na and ~(86)Rb and Ionic Conduction in Sodium-Rubidium Borate Glasses: Temperature and Composition Dependence

摘要

Measurements of ~(22)Na and ~(86)Rb tracer diffusion were made in a series of 0.2[X Na_2O · (1-X) Rb_2O] · 0.8 B_2O_3 ion-conducting glasses as functions of composition (X=0.0, 0.2, 0.4, 0.6, 0.8, and 1.0) and temperature (504-654 K). At constant temperature the ~(86)Rb diffusivity D_(Rb) decreases nearly exponentially with the Rb-content in the whole mixed-alkali range. The ~(22)Na diffusivity D_(Na)~* decreases on the Na-rich side with decreasing Na-content, but reaches an almost composition-independent plateau value on the Rb-rich side. This points to a major difference between the diffusion of the larger Rb and the smaller Na ions. D_(Na)~* and D_(Rb)~* are equal near X=0.2, which we denote as diffusivity-crossover composition. This composition is almost temperature independent. The pre-exponential factors as a function of the activation enthalpies obtained from the Arrhenius-type temperature dependencies of the tracer diffusivities obey the Meyer-Neldel rule. We compare the tracer diffusion results with ionic conductivity data obtained earlier in our laboratory The tracer transport is found to be slower than the charge transport, which is expressed by a Haven ratio smaller than unity. The common Haven ratios of the Narich Na-Rb borate glasses are independent of temperature, however a decrease of the Haven ratio with decreasing temperature is observed for the Rb-rich Na-Rb borate glasses.