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

Abstract

Activation energies of the diffusion of alkaline earth ions in silicate glasses of the composition A 2O 2BO 4SiO 2 have been compiled recently, where A is an alkali and B an alkaline earth metal. A pronounced dependence on the ratio r B/ r A of radii of alkaline earth ions B 2+ 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 r B/ r A<1 when compared with the one for r B/ r A>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.