Two new members in the quaternary Cs–Ag–As–S family with different arrangements of Ag–S and As–S asymmetric building units: syntheses, structures, and theoretical studies

Abstract

New functional materials of As-based multinary chalcogenidometalates have received special attention due to their different oxidation states and flexible building blocks. In this work, two new quaternary thioarsenates(III), namely Cs₂Ag₂As₂S₅ (I) and Cs₃AgAs₄S₈ (II), have been obtained by a simple surfactant–thermal technique. Their structures were determined on the basis of single-crystal X-ray diffraction; compound I belongs to the triclinic space group P (no. 2) [a = 7.49(2) Å, b = 9.30(2) Å, c = 9.94(2) Å, α = 63.28(3)°, β = 82.11(3)°, γ = 87.78(3)°, V = 612.4(2) ų and Z = 2], whereas compound II crystallizes in the monoclinic space group C2/c (no. 12) [a = 27.76(6) Å, b = 6.98(2) Å, c = 19.68(4) Å, β = 109.85(3)°, V = 3586.7(2) ų and Z = 8]. Both structures are shown to feature two-dimensional (2D) layers with different arrangements of Ag–S and As–S asymmetric building units (ABUs). In compound I, the layer is constructed from vertex-sharing dimeric [As₂S₅] units linked by tetrameric [Ag₄S₈] moieties. The layered structure in compound II is formed by 2₁ helical chains based on [As₄S₉] groups which are further interconnected by tetrahedral [AgS₄] chains. The analysis of the quaternary X–Ag–As–S (X = cations) system shows that various combination ways of Ag–S and As–S ABUs in these sulfides can be attributed to the cation-cation repulsion and stereochemically active lone-pair effect. The UV-vis/NIR absorption spectrum indicates that the optical band energy (E_g) of compounds I and II is 2.48 eV and 2.24 eV, respectively. Moreover, theoretical calculations indicate that the As–S and Ag–S bonding states determine the optical properties of the title compounds. This work not only enriches the structural chemistry of thioarsenates, but also opens up a new avenue for exploring novel multinary chalcogenidometalates.