Ion transport and structure in chalcogenide glasses

Ion-conducting chalcogenide glasses have been chosen to illustrate three specific electrical effects in glasses and their relationship with the glassy structure. Firstly, the variation of conductivity with the mobile cation content was investigated for glasses from the systems Ag 2 S–GeS 2 , Ag 2 S–As 2 S 3 and x Ag 2 S–60GeS–(40 − x )GeS 2 . Electrical conductivity and field emission scanning electron microscopy measurements, carried out over an extremely large composition range, show clearly the role of the macroscopic structure in the electrical properties. Two conductivity regimes are observed and the change from the low conductivity regime to the high conductivity regime in the phase separated glasses Ag 2 S–GeS 2 and Ag 2 S–As 2 S 3 occurs when the regions containing the Ag-rich phase start to connect. Secondly, a mixed glass former effect was observed in the glassy system 0.3Li 2 S–0.7[(1 − x )SiS 2 – x GeS 2 ] (0 ≤ x ≤ 1), corresponding to an enhancement of the ionic conductivity for the central region 0.5 ≤ x ≤ 0.64. Structural investigations by Raman and small angle x-ray scattering (SAXS) techniques indicate that the glasses from the limiting composition ranges are homogeneous while glasses belonging to the central region are phase separated into two compounds with compositions close to GeS 2 and Li 2 SiS 3 . Finally, glasses with composition 0.5[(1 − x )Rb 2 S− x Ag 2 S]–0.5GeS 2 show clear manifestations of a mixed cation effect. SAXS measurements show that the glasses are homogeneous and extended x-ray absorption fine structure investigations indicate that each mobile cation, Ag + or Rb + , maintains its own specific environment. On the other hand, Raman spectra indicate a non-linear structural change of the glass matrix upon cation mixing with a rearrangement of the local GeS 4 tetrahedra to form chains.