A revised view on the mixed-alkali effect in alkali borate glasses

The mixed-alkali effect (MAE) of the ionic conductivity is traditionally defined as a negative deviation of the dc conductivity from that of an 'ideal glass' obtained according a linear mixing rule. An alternative definition is proposed in this paper which relates the conductivity of a mixed-alkali glass to the conductivity-sum of two single-alkali glasses with total alkali concentrations equal to the corresponding partial alkali concentrations of the mixed-alkali glass. A new reference glass denoted as independent component glass (ICG) is introduced. In contrast to the 'ideal glass' the conductivity of a real mixed-alkali glass is larger than the conductivity of the respective ICG resulting in a positive MAE. This fact manifests an enhancement of the conductivity when two single-alkali glasses are combined to a mixed-alkali glass. The conductivities and the tracer diffusivities of the mixed-alkali- and the corresponding single-alkali glasses are compared from this point of view. A subnetwork diffusion concept (SDC) is proposed to describe the diffusion dynamics in Na-Rb-borate mixed-alkali glasses.