The Layered Semiconductor Cu(Sb 2 S 3 )[AlCl 4 ]

Sb 2 S 3 and CuCl were reacted in the ionic liquid [BMIm]Cl ⋅ 4.4AlCl 3 (BMIm=1‐ n ‐butyl‐3‐methylimidazolium) at 200 °C. Upon cooling to room temperature, orange‐red colored, air‐sensitive crystals of Cu(Sb 2 S 3 )[AlCl 4 ] precipitated. X‐ray diffraction on a single‐crystal revealed an orthorhombic crystal structure, in which cationic [Cu(Sb 2 S 3 )] + layers are separated by tetrahedral [AlCl 4 ] − anions. The uncharged Sb 2 S 3 partial structure consists of one‐dimensional strands with covalent Sb−S single bonds. The copper(I) cation is coordinated by three sulfur atoms and by one of chlorine atoms of the [AlCl 4 ] – anion. An optical band gap of 2.14 eV was deduced from UV/Vis spectra. In very good agreement, a band gap of 2.07 eV results from DFT‐based calculations involving a new implementation of the bifunctional formalism for the exchange energy. By treatment with 0.1 molar hydrochloric acid, AlCl 3 was leached from Cu(Sb 2 S 3 )[AlCl 4 ] yielding a compound with the presumed composition Cu(Sb 2 S 3 )Cl. Hydrolysis at higher pH resulted in Cu 2–x S and Sb 4 O 5 Cl 2 .