Ionic transport crossover in mixed conducting chalcogenide glasses detected by Te-125-Mossbauer spectroscopy

Te-125-Mossbauer spectroscopy has been used to verify whether the observed ionic transport crossover for the Ag-As-Se-Te glasses with a silver content of 15 at.% at approximately equimolar Se/Te ratio is accompanied by variat ions in the hyperfine interaction parameters produced by corresponding chan ges in the microstructural organization of the glass network. We found that an unusual decrease of the quadrupole splitting with decreasing tellurium fraction, r = Te/(Se + Te) < 0.5, is caused by the combined effect of incre asing Te-Ag(As) interatomic distances and a nonrandom substitution of multi ple Se sites by Te with a preferential occupation of those with at least on e Ag nearest neighbour. The latter process leads also to a step-like decrea se in the isomer shift at r less than or equal to 0.35. The assumed expansi on of the glass network at smaller r and the contrasting network contractio n for the Te-rich glasses are consistent with the ionic transport crossover . (C) 1999 Elsevier Science B.V. All rights reserved.