Ion transport and structure in chalcogenide glasses

GeS2 based glasses have been chosen to illustrate three different specific electrical effects in glasses and their relationship with the glassy structure. 1) Variation of the conductivity with the mobile cation content : the silver chalcogenide glass family Ag2S-F, with F = GeS2, As2S3, GeS-GeS2 has been chosen as an example. Electrical and FE-SEM measurements carried out over an extremely large composition range show clearly the role of the macroscopic structure on electrical properties. Glasses with F = GeS2 or AS(2)S(3) are phase separated with a percolation threshold at about 5 at % in Ag as shown by FE-SEM measurements. The electrical conductivity shows two different regimes below and above the percolation threshold. 2) Mixed glass former effect : a mixed glass former effect has been observed in the glassy system 0.3 Li2S0.7[(1-X)SiS2 similar to xGeS(2)] where 0 less than or equal to x less than or equal to 1. It corresponds to an enhancement of ionic conductivity for 0.5 less than or equal to x less than or equal to 0.64. Structural investigations by Raman and SAXS techniques indicate that the glasses from the limiting composition ranges are homogeneous while glasses belonging to the central region are phase separated with two phases of compositions close to GeS2 and Li2SiS3. 3) Mixed cation effect: glasses with composition 0.5 [(1-x)Rb2S-xAg(2)S]-0.5 GeS2 (0 less than or equal to x less than or equal to 1) show clear manifestations of mixed cation effects (minimum in conductivity and Tg variations, maximum in the activation energy variation). SAXS measurements show that the glasses are homogeneous. EXAFS investigations indicate that each mobile cation maintains its own specific environment. On the other hand, Raman and IR spectra indicate a non-linear structural changes of the glass matrix upon cation mixing with rearrangements of the local GeS4 tetrahedra chaining.