Ion transport and structure in chalcogenide glasses

Abstract

Ion-conducting chalcogenide glasses have been chosen to illustrate three specificelectrical effects in glasses and their relationship with the glassy structure.Firstly, the variation of conductivity with the mobile cation content wasinvestigated for glasses from the systems Ag2S–GeS2, Ag2S–As2S3 andxAg2S–60GeS–(40 − x)GeS2.Electrical conductivity and field emission scanning electron microscopymeasurements, carried out over an extremely large composition range, show clearlythe role of the macroscopic structure in the electrical properties. Two conductivityregimes are observed and the change from the low conductivity regime to the highconductivity regime in the phase separated glasses Ag2S–GeS2 and Ag2S–As2S3occurs when the regions containing the Ag-rich phase start to connect. Secondly, amixed glass former effect was observed in the glassy system 0.3Li2S–0.7[(1 − x)SiS2–x GeS2] (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) techniquesindicate that the glasses from the limiting composition ranges are homogeneous whileglasses belonging to the central region are phase separated into two compoundswith compositions close to GeS2 and Li2SiS3. Finally, glasses with composition0.5[(1 − x)Rb2S−xAg2S]–0.5GeS2 show clear manifestations of a mixed cation effect. SAXSmeasurements show that the glasses are homogeneous and extended x-rayabsorption fine structure investigations indicate that each mobile cation, Ag+ orRb+, maintains its own specific environment. On the other hand, Raman spectraindicate a non-linear structural change of the glass matrix upon cationmixing with a rearrangement of the local GeS4 tetrahedra to form chains.