Conductivity percolation transition of Agx(Ge0.25Se0.75)100?x glasses

The ionic conductivity of several chalcogenide glasses increases abruptly with mobile ion addition from values typical of insulating materials (10^?16–10^?14 Ω^?1 cm^?1) to values of fast ionic conductors (10^?7–10^?1 Ω^?1 cm^?1). This change is produced in a limited concentration range pointing to a percolation process. In a previous work M. Kawasaki, J. Kawamura, Y. Nakamura, M. Aniya, Solid State Ionics 123 (1999) 259] the transition from semiconductor to fast ionic conductor of Ag_x(Ge_0.25Se_0.75)_100?x glasses was detected at x¹ ? 10 at.% in the form of a steep change in the conductivity. Ag_x(Ge_0.25Se_0.75)_100?x glasses with x ? 25 at.%, prepared by a melt quenching method, are investigated by impedance spectroscopy in the frequency range 5 Hz–2 MHz at different temperatures, T, from room temperature to 363 K and by DC measurements at room temperature. The conductivity of the glasses, σ, was obtained as a function of silver concentration and temperature. For x ? 10 at.% our results are in agreement with those reported by Kawasaki et al. M. Kawasaki, J. Kawamura, Y. Nakamura, M. Aniya, Solid State Ionics 123 (1999) 259]. The percolation transition was observed in the range 7 ? x ? 8. The temperature dependence of the ionic conductivity follows an Arrhenius type equation σ = (σ_o/T) · exp(?E_σ/kT). The activation energy of the ionic conductivity, E_σ, and the pre-exponential term, σ_o, are calculated. The results are discussed in connection with other chalcogenide and chalcohalide systems and linked with the glass structures.