Information on the absolute length scales of ion transport processes in glasses from electrical conductivity and tracer diffusion data

We show that absolute values for the time-dependent mean-square displacemen t of mobile ions in glasses can be obtained from a combined analysis of ele ctrical conductivity and tracer diffusion data. By applying this method to sodium germanate glasses of different compositions, we find that the distan ces the ions cover in characteristic stages of the transport process increa se considerably with decreasing sodium oxide content. At low sodium oxide c ontents, we observe a linear relation between the transport distances and t he interionic separation distances, while this relation is lost at high sod ium oxide contents. The same is found to be true for other alkali ion condu cting glassy systems. The analysis of this effect is important for discrimi nating between the influence of the network structure and of the interionic Coulomb interactions on the ion dynamics. (C) 1999 Elsevier Science B.V. A ll rights reserved.