ION TRANSPORT IN CHALCOGENIDE GLASSES: DYNAMICS AND STRUCTURAL STUDIES

In this paper we attempt to summarise the state of our knowledge on ionic conductive chalcogenide glasses. The silver chalcogenide glass family has been chosen as an example. Measurements of Ag tracer diffusion experiments (DAg) and electrical measurements (σi) carried out over an extremely large composition range elucidate the dc conductivity which depends on diffusion over long distances. The results show three distinctly different transport regimes: i) below the percolation threshold at xc ≈ few silver ppm the glasses are ionic insulators, ii) just above the percolation threshold (xc < x < 1-3 at % Ag) σi and DAg are very well described by a modified geometrical percolation model using a single parameter, the critical temperature To : σi (x) ∝ x , DAg (x) ∝ x , E(x) ∝ k To log (x/xc). This critical temperature, reflecting interconnectivity of "infinite" percolation clusters embedded in the glassy matrix depends on the structural organisation of the host matrix, iii) far above the percolation threshold the Ag ion transport depends on the Ag content but not on the host matrix. Accordingly only at low mobile ion content structure or, more precisely, dimensionality of the vitreous matrix seems to play a role in ionic transport properties of glasses. Concerning the ac conductivity the complete conductivity spectra (log σ(ω) vs log f (Hz)), obtained in a very broad temperature and frequency ranges can be perfectly fitted by σ(ω) = σdc + Aω + Bω + Cω. Thus at high frequencies (GHz range) and high temperatures the superlinear frequency dependence of the conductivity recently evoked to describe the data can be discarded. Moreover the existence of a high frequency plateau postulated by many models can be questioned.