Mechanism of mixed alkali effect in silicate glass/liquid: Pathway and network analysis

The mixed alkali effect in silicate glass/liquid causes two anomalies: a decrease in the self-diffusivity of alkali ions and a decrease in shear viscosity. Previous studies using molecular dynamics (MD) simulations proposed that the decrease in diffusivity of alkali ions is the result of pathway blocking. However, the actual statistics of the pathway have never been presented. In this study, we revisited the mixed alkali effect in sodium–potassium silicate glass/liquid using MD simulations. The distinct part of van Hove's function indicates that site exchange between different alkali species is significantly slower than between the same species. By introducing simplex sphere analysis and its cluster statistics with varying compositions, we clarified the pathway disconnection caused by alkali mixing. Moreover, we analyzed the geometry and topology of the corner-shared network of the SiO4 tetrahedra precisely. Both the geometry and topology change linearly with advancing alkali substitution, whereas the calculated shear viscosities have a minimum in the mixed alkali composition. Consequently, a method to quantitatively analyze the changes in relaxation dynamics is required to clarify the viscosity anomaly in mixed alkali liquids.