Cu 22Bi 12S 21Cl 16—A mixed conductor with fast one-dimensional copper(I) ion transport

Abstract

Melting reactions of Cu, CuCl, S, and Bi 2S 3 yield black, shiny needles of Cu 22(1)Bi 12S 21(1)Cl 16(1). The compound decomposes peritectically at 649(5) K. Oxidation state +I of the copper atoms is supported by Cu– K-XANES. The compound crystallizes in the hexagonal space group P6/ m with a=2116.7(7) pm and c=395.17(5) pm. Seven anions coordinate each of the two independent bismuth cations in the shape of mono-capped trigonal prisms. These polyhedra share edges and faces to form trigonal and hexagonal tubes running along [0 0 1]. The hexagonal tubes are centered by chloride ions, which are surrounded by disordered copper cations. The majority of copper cations are distributed over numerous sites between the tubes. The Joint Probability Density Function (JPDF) reveals a continuous pathway along [0 0 1]. The high mobility of the copper cations along [0 0 1] was demonstrated by impedance spectroscopy and DC polarization measurements on single crystals. The ionic conductivity at 450 K is about σ ion=0.06 S cm −1, and the activation energy for Cu + ion conduction is E a=0.44 eV. The chemical diffusion coefficient of copper is in the order of D cu δ =10 19 cm −3 at 420 K. The electronic band gap (p-type conductor) was determined as E g=0.06 eV. At room temperature the thermal conductivity of a pressed pellet is about κ=0.3 W K −1 m −1 and the Seebeck coefficient is S=43 μV K −1.