EXAFS studies of Cu+ ion conducting and semiconducting copper chalcogenide and chalcohalide glasses

The short-range order in semiconducting xCu(2)Se .(1 - x)As2Se3 and mixed conducting xCuI .(1 - x)As2Se3 glasses has been investigated using Extended X-ray Absorption Fine Structure (EXAFS) measurements at the Cu, As and Se K-edges. A non-tetrahedral local coordination of copper, either by Se or by Se and I, has been found in these glassy systems. The pyramidal AsSe3 units remain essentially intact on introducing Cu2Se or CuI. However, the average selenium coordination number, NSe-Y where Y = As or Cu, increases with x from 2.0 to 2.5-2.6. Simple stoichiometric relationships account for the observed changes in NSe-Y for the Cu2Se-As2Se3 glasses, which suggest that,at a nearest neighbour level, the average selenium environment consists of two relatively weakly interacting components coming from vitreous As2Se3 and a hypothetical glassy Cu2Se. On the contrary, the EXAFS results indicate that CuI is dispersed in the glass network on a molecular level and interacts with the host matrix. Favourable conductivity pathways, associated with CuI, are suggested to appear in the glass structure, which explain the improved Cu+ ion conduction and copper diffusion. (C) 1998 Elsevier Science B.V, All rights reserved.