On the mobility of alkaline earth ions in mixed alkali alkaline earth silicate glasses

Activation energies of the diffusion of alkaline earth ions in silicate glasses of the composition A2O 2BO 4SiO2 have been compiled recently, where A is an alkali and B an alkaline earth metal. A pronounced dependence on the ratio rB/rA of radii of alkaline earth ions B2+ and alkali ions A+ has been detected. A minimum occurs at a ratio of unity and the increase of the activation energies is much steeper for rB/rA<1 when compared with the one for rB/rA>1. This size effect of alkaline earth mobility can be explained qualitatively and quantitatively by a model of the mixed alkali effect, where the ionic mobility is affected by both a distribution of site energies and the packing density of oxygen.