Ionic transport in mixed sodium rubidium germanate glasses

Abstract

Ionic conductivity measurements have been made as a function of temperature and Na/Rb ratio on four series of x(Na, Rb) 2O: (1− x)GeO 2 glasses where x≈0.09, 0.15, 0.19 and 0.29. The three series with x⩾0.15 exhibit the common characteristic of mixed alkali glasses; a maximum in the enthalpy for conductivity, H m(max) for glasses with Na ( Na + Rb) ≈ 0.5. The magnitude of the mixed alkali effect, as measured by the difference between H m (max)and the enthalpies for conductivity of the single alkali glasses of the same series, could not be correlated with changes in the glass network structure or the type and concentration of charge compensating centers in the glass network. The pre-exponential factors for the conductivity are analysed to obtain values of the physical correlation factor or percolation efficiency, f t. In each of the four series f t was a maximum at Na ( Na + Rb) ≈ 0.5 and based on the changes in the percolation effeciency of the mixed alkali glasses a model is proposed to explain the mixed alkali effect.