The mixed alkali effect revisited: the importance of ion–ion interactions

Abstract

The most dramatic mixed alkali (MA) effect occurs in the dilute foreign alkali region where analysis has shown that one foreign alkali appears to immobilize a large number of host ions. This large number can now be rationalized, if ion–ion interaction is taken into consideration, by a similar situation in an analogous interacting system of neutral molecules. In this case molecular dynamics data have shown that a frozen particle can immobilize not just its nearest neighbor particles, but that the effect propagates to longer distances. The immobilized ions account for the excess low frequency response and the large dielectric strength of MA glasses. The electric modulus suppresses this excess low frequency response and accentuates the mobile ion contribution. The large decrease in the concentration of mobile ions when mixing two alkalis and the concomitant reduction of ion–ion interactions between them leads to an explanation of the narrowing of the electric modulus dispersion in high alkali content glasses. The non-additivity of conductivities and the opposite behavior of the broadening of the electric modulus dispersion when mixing two alkalis in glasses with very low total alkali content is due to partial spatial blockage of transport of the more mobile ions by regions containing the less mobile ions. The mechanisms proposed for the MA effect work also for crystalline ionic conductors as well.