110Ag tracer diffusion study of percolation transition in Ag2S-As2S3 glasses

Silver tracer diffusion study of homogeneous Ag2S-As2S glasses has been realised over an extremely wide composition range covering nearly five orders of magnitude in the silver concentration from 4 X 10(-4)% (4 ppm) to 33.3 at.% Ag. Three characteristic composition domains with drastically different ion transport properties have been found. (i) Below the percolation threshold at.x(c) approximate to 3 x 10(-3) at.% Ag, the diffusion coefficients D-Ag and activation energy E-d do not depend on the silver content and similar to those in pure As2S3; the silver ion transport number t(Ag+) is 0.10-0.14. (ii) In the critical region just above the percolation threshold (x=0.012-1.2 at.% Ag), the glasses are neatly pure Ag+ ion conductors (t(Ag+) >0.8), the ionic conductivity sigma (1)(x) and diffusion coefficients D-Ag(x) exhibit a power-law composition dependence: sigma (j) proportional tox' and D-Ag proportional tox(t-1) with temperature-dependent power-law exponent t proportional toT(0)/T. (iii) At x>15 at.% Ag, a well-known rather exponential err increase of sigma (i)(x) and D-Ag(x) is observed with strongly correlated Ag+ ion motion. (C) 2000 Elsevier Science B.V. All rights reserved.