Percolation transition in Ag-doped chalcogenide glasses: comparison of classical percolation and dynamic structure models

Homogeneity and electrical characteristics of Ag2S-As2S3 glasses with silver concentrations ranging between 0.012 and 9.8 at.% have been investigated. It was found that diluted vitreous alloys (x = 0.012-1.2 at.% Ag) appear to be homogeneous according to scanning electron microscopy studies. Their electrical conductivity increases by a factor of 40 with corresponding decrease of the activation energy from 0.88 eV to 0.78 eV. The conductivity isotherm in this concentration range exhibits a power law composition dependence (sigma(x)proportional to x(t)), which is characteristic of percolation in the critical region just above the percolation threshold. In contrast to classical geometric percolation, t is temperature-dependent (t proportional to T-1). The power-law exponent at room-temperature (t(25) = 0.78(6)) is smaller by a factor of 2.5 than, that for a Ag2S-GeS-GeS2 glassy system in the same Ag concentration range and indicates that the electrical conductivity occurs within the framework of 2D percolation contrary to silver germanium sulphide glasses. Modified geometrical percolation predicts this decrease in t, which is not explained by the dynamic structure model. (C) 1998 Elsevier Science B.V. All rights reserved.