Deciphering the Mixed-Alkali effect in supercooled oxide liquids: Results from dynamic and thermodynamic measurements

The fundamental nature of the mixed-alkali effect (MAE) on viscosity, fragility index and glass transition of supercooled borate and phosphate liquids is studied using a combination of dynamic and thermodynamic measurements. The negative deviation of T_g from linearity in mixed-alkali borates is found to be always the largest when two distinct alkalis are combined in equal proportions, consistent with the prediction of the matrix-mediated coupling model. However, this deviation is shown to vary nonmonotonically with the alkali size difference, which is attributed here to a balance between the strength and the reorientation probability of the mechanical dipoles associated with the hopping of alkalis to unlike sites. On the other hand, isobaric heat capacity measurements of mixed-alkali phosphate glasses and supercooled liquids indicate that the negative deviation of the liquid fragility index m from linearity may not be related to the configurational entropy of mixing of the alkalis. It is hypothesized that the bond scission-renewal dynamics of the network responsible for the viscous flow is catalyzed by the reorientation process of the alkali mechanical dipoles in mixed-alkali liquids, which results in a lowering of the activation energy of the former. This lowering of activation energy is manifested in a significant lowering of m of a mixed-alkali liquid, compared to its single-alkali end members that lack this catalytic feedback. The relative importance of this catalytic contribution diminishes at higher temperatures as the thermal contribution to the activation barrier crossing begins to dominate and thus the MAE on viscosity eventually disappears above a critical temperature.