Hardness of silicate glasses: Atomic-scale origin of the mixed modifier effect

The origin of the various manifestations of the mixed modifier effect in silicate glasses remains poorly understood. Here, based on molecular dynamics simulations, we investigate the origin of a negative deviation from linearity in the hardness of a series of mixed alkaline earth aluminosilicate glasses. The minimum of hardness is shown to arise from a maximum propensity for shear flow deformations in mixed compositions. We demonstrate that this anomalous behavior originates from the existence of local structural instabilities in mixed compositions arising from a mismatch between the modifiers and the rest of the silicate network. Overall, we suggest that the mixed modifier effect manifests itself as a competition between the thermodynamic driving force for structural relaxation and the kinetics thereof.