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 Agx(Ge0.25Se0.75)100−x glasses was detected at x∗ ≅ 10 at.% in the form of a steep change in the conductivity. Agx(Ge0.25Se0.75)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 = (σo/T) · exp(−Eσ/kT). The activation energy of the ionic conductivity, Eσ, and the pre-exponential term, σo, are calculated. The results are discussed in connection with other chalcogenide and chalcohalide systems and linked with the glass structures.