The mixed modifier effect in ionic conductivity and mechanical properties for xMgO-(50-x)CaO-50SiO2 glasses

The mixed-modifier effect (MME), i.e., the tendency of the physical properties of a glass to deviate from the law of additivity when multiple network-modifying components are included, can cause large improvements in the mechanical properties of glasses. However, previous studies of the MME have been largely limited to systems containing alkali metal modifiers, and the origins of the MME for mechanical properties remain unclear. Here, a series of glasses with purely mixed alkaline earth modifiers, xMgO-(50 −x)CaO-50SiO2, and prepared and used to verify the existence of the mixed-alkaline-earth effect (MAEE) in ionic conductivity and mechanical properties. While the deviations from linearity for the MAEE were not as large as is typical for the mixed-alkali effect (MAE), they were still significant (up to 30%). Unexpectedly, the ionic conduction in these glasses was found to deviate significantly from Arrhenius behaviour, with the activation energy decreasing at elevated temperature. Examination of the MME in mechanical properties showed that the activation energy for ion conduction was correlated with shear stiffness, but not compressibility, indicating that the shear modulus may serve as a reasonable predictor of the strain energy required for ion hopping during conduction. The connectivity of the glass network, as determined by Raman spectroscopic analysis, was used to relate the MME in static properties and dynamic properties.