Preparation, structural, Raman and impedance spectroscopic characterisation of the silver ion conductor (AgI)2Ag3SbS3

Pale yellow (AgI)2Ag3SbS3 was synthesized by the reaction of stoichiometric amounts of AgI and Ag3SbS3 (2 ∶ 1) at 683 K. It is air stable for several months. The crystal structure was determined at different temperatures in the range from 173 K to 573 K by single crystal X-ray diffraction. (AgI)2Ag3SbS3 crystallizes in the orthorhombic system, space group Pnnm (no. 58) with a = 10.9674(8) Å, b = 13.5200(12) Å, c = 7.7460(5) Å, V = 1156.3(5) Å3, and Z = 4 (data at 298 K). The title compound is isotypic with (CuI)2Cu3SbS3, at least for the positions of I, Sb, and S. The silver atoms are highly disordered and therefore their displacement parameters were refined using a Gram-Charlier non-harmonic development. No phase transition is observed between 173 K and the melting point at 720 K (DSC, onset temperature). The distribution of silver changes drastically with temperature and the localization of silver increases at low temperature. A high ionic conductivity is observed in combination with a pronounced disorder of the silver atoms. Impedance spectroscopic measurements reveal specific conductivity data of σ = 8.15 × 10−5 Ω−1 cm−1 at 332 K and of σ = 1.52 × 10−3 Ω−1 cm−1 at 478 K. The activation energy is EA = 0.29 eV. Raman spectra are dominated by the stretching modes of the [SbS3]3− units at 357, 327 and 316 cm−1 at room temperature