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

Abstract

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) ų, 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 E_A = 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.