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 A(2)O 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 B2+ and alkali ions A(+) has been detected. A minimum occurs at a ratio of unity and he 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. (C) 2003 Elsevier B.V. All rights reserved.