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

Abstract

The ionic conductivity of several chalcogenide glasses increases abruptly with mobile ion addition from values typical of insulating materials (10−16–10−14Ω−1cm−1) to values of fast ionic conductors (10−7–10−1Ω−1cm−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∗≅10at.% in the form of a steep change in the conductivity. Agx(Ge0.25Se0.75)100−x glasses with x⩽25at.%, prepared by a melt quenching method, are investigated by impedance spectroscopy in the frequency range 5Hz–2MHz at different temperatures, T, from room temperature to 363K 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⩾10at.% 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.