Percolation transition in Ag-doped germanium chalcogenide-based glasses: Conductivity and silver diffusion results

Conductivity and silver diffusion measured using a Ag-110m tracer have been investigated in Ag-Ge-S and Ag-Ge-Sb-Se glasses with silver concentration ranging from 0.008 to 25 at.% Ag. It has been found that the room-temperature conductivity in both systems increases by 9.0-9.5 orders of magnitude with increasing silver content, and its activation energy decreases from similar to 1 to 0.4 eV. Accordingly, the silver tracer diffusion coefficient at 298 K increases by 5.0-5.5 orders of magnitude with similar decrease of the diffusion activation energy. A comparison of the conductivity and silver diffusion results clearly shows that the ionic transport is predominant in the two systems, even at lowest Ag concentrations. The Haven ratio, H-R, decreases with increasing silver content: extremely diluted glasses (0.008-0.1 at.% Ag) exhibit H-R approximate to 1; Ag-rich vitreous alloys are characterized by H-R = 0.2-0.4. The composition dependencies of the ionic conductivity, sigma(i), and silver tracer diffusion coefficient, D-Ag, exhibit two drastically different transport regimes at low (less than or equal to 2-5 at.%) and high (> 10 at.%) silver concentrations, A power-law composition dependence of sigma(i) and D-Ag over 2.5 orders of magnitude in the Ag concentration and 3.5-5.0 orders of magnitude in the ionic conductivity (2-3 orders of magnitude in the diffusion coefficient) is observed at low silver concentrations. This transport regime is attributed to percolation in the critical region just above the percolation threshold. Recent theoretical considerations (the dynamic structure model and statistical (occupation) effects on percolative ionic conduction) are also in good agreement with experimental findings, After essential structural transformations of the glass network on the short- and intermediate-range scales at higher silver content (> 10 at.%), the ionic transport is not caused any more by percolation, i.e., it becomes network-dependent with a strongly correlated motion of the Ag+ ions.