Die Serie caesiumhaltiger Thioarsenate(V) der Lanthanoide vom Formeltyp Cs3Ln[AsS4]2 mit Ln = La–Nd und Sm

Abstract Cesium-containing thioarsenates(V) of lanthanoids with the composition Cs3Ln[AsS4]2 can be prepared for Ln = La–Nd and Sm by the oxidation of the lanthanoid metals in the presence of orpiment (As2S3), cesium sesquisulfide (Cs2S3) and elemental sulfur (S). The reaction mixtures were handled under argon atmosphere and heated in fused silica ampoules to a moderate reaction temperature of 500 °C. Single crystals of this Cs3Ln[AsS4]2 series appear as yellowish, transparent rods that are not stable to moist air and water. Their monoclinic crystal structure was determined from single-crystal X-ray diffraction data. The cesium-containing thioarsenates(V) Cs3Ln[AsS4]2 with Ln = La–Nd and Sm crystallize isotypically with Rb3Sm[PS4]2 in the non-centrosymmetric monoclinic space group P21 with Z = 2. The lattice parameters range between a = 997–1015 pm, b = 699–703 pm, c = 1192–1195 pm and β = 90.2–90.4°. There are two crystallographically different As5+ cations and one unique Ln3+ cation present, all coordinated by sulfur anions. Tetrahedral [AsS4]3− anions form two functionally different units working either as bridging units between three Ln3+ cations or as terminal ligands for Ln3+. [LnS8]13− polyhedra form ∞ 1 { [ Ln S 2 / 2 v S 6 / 1 t ] 11 − } ${}_{\infty }^{1}\left\{{[\mathit{Ln}{\mathrm{S}}_{2/2}^{\mathrm{v}}{\mathrm{S}}_{6/1}^{\mathrm{t}}]}^{11-}\right\}$ chains propagating along [010], which are separated by Cs+ cations with coordination numbers as high as C.N. = 9, 10 and 11. Surprisingly, the eighth interatomic distance d(Ln–S3) does not follow the lanthanoid contraction, but increases with decreasing ionic radius, changing the coordination polyhedron of Ln3+ from a bicapped to a monocapped trigonal prism upon the transition from Nd to Sm, and causing a structural break in the crystal structure of this series. The results of Raman spectroscopic measurements on single crystals of Cs3La[AsS4]2 have confirmed the presence of [AsS4]3− tetrahedra as structural building blocks.