Homogeneity of modifier ion distributions and the mixed alkaline earth effect in MgO–CaO–SiO2 silicate glasses using molecular dynamics

Oxide glasses containing a mixture of modifier ions have a wide range of applications, and it is often assumed that these modifier ions mix homogenously in the melt. Silicate glasses are known for their significant industrial importance and are often doped with magnesium and calcium modifier ions to utilise their properties for applications. This study investigated the structural and dynamical impact imposed on a glass by the presence of two different types of alkaline earth cations. This was achieved through increasingly substituting magnesium for calcium in xMgO–(50–x)CaO–50SiO2 glasses which were modelled computationally using classical molecular dynamics simulation (MD). A mixed alkaline earth effect (MAEE) was found to cause the suppression of the modifier ion diffusivity. An absence of any significant deviation from the expected trends in the structural investigations confirmed that the MAEE is not a structural effect. This MAEE was therefore analogous to the mixed alkali effect (MAE), and was most prominent for roughly equimolar mixtures of calcium and magnesium ions. Interestingly, there was also evidence of the diffusivity being enhanced when only a small amount of a second alkaline earth ion was added.