The relevance of site energy distribution for the mixed alkali effect

The mixed alkali effect (MAE) can be explained as an interplay between netw ork modifications induced by alkali ions of different size and an energy di stribution of sites with a preferential occupancy of the low energy sites b y the smaller cations. The underlying physical assumptions of this concept are discussed in the present paper in more detail and they are compared wit h results of Monte Carlo simulations of diffusion and the Anderson-Stuart m odel. It will be shown that the new concept is actually hidden in the Ander son-Stuart model. Applying the new concept to experimental data has been do ne so far with a box-type distribution of site energies. Besides the oversi mplification, a major drawback of this distribution is that the diffusion c oefficient of the smaller and minority cation is underestimated. It will be shown in the present study that by a different choice of distribution the concentration dependence of the diffusivity of the minority cations can be understood. In addition some results in quaternary glasses will be explaine d by a more general distribution function. (C) 2001 Elsevier Science B.V. A ll rights reserved.