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 conc entration 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 or ders of magnitude with increasing silver content, and its activation e nergy decreases from similar to 1 to 0.4 eV. Accordingly, the silver t racer diffusion coefficient at 298 K increases by 5.0-5.5 orders of ma gnitude with similar decrease of the diffusion activation energy. A co mparison of the conductivity and silver diffusion results clearly show s that the ionic transport is predominant in the two systems, even at lowest Ag concentrations. The Haven ratio, H-R, decreases with increas ing silver content: extremely diluted glasses (0.008-0.1 at.% Ag) exhi bit 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 com position 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 c onductivity (2-3 orders of magnitude in the diffusion coefficient) is observed at low silver concentrations. This transport regime is attrib uted to percolation in the critical region just above the percolation threshold. Recent theoretical considerations (the dynamic structure mo del and statistical (occupation) effects on percolative ionic conducti on) are also in good agreement with experimental findings, After essen tial structural transformations of the glass network on the short- and intermediate-range scales at higher silver content (> 10 at.%), the i onic transport is not caused any more by percolation, i.e., it becomes network-dependent with a strongly correlated motion of the Ag+ ions.