The series of cesium-containing thioarsenates(V) of lanthanoids of the formula type Cs${}_3$Ln[AsS${}_4$]${}_3$ with Ln = La-Nd and Sm

Cesium-containing thioarsenates(V) of lanthanoids with the composition Cs${}_3$Ln[AsS${}_4$]${}_2$ can be prepared for Ln = La-Nd and Sm by the oxidation of the lanthanoid metals in the presence of orpiment (As${}_2$S${}_3$), cesium sesquisulfide (Cs${}_2$S${}_3$) 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 Cs${}_3$Ln[AsS${}_4$]${}_2$ 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) Cs${}_3$Ln[AsS${}_4$]${}_2$ with Ln = La-Nd and Sm crystallize isotypically with Rb${}_3$Sm[PS${}_4$]${}_2$ in the non-centrosymmetric monoclinic space group P2${}_1$ 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 As${}^{5+}$ cations and one unique Ln${}^{3+}$ cation present, all coordinated by sulfur anions. Tetrahedral [AsS${}_4$]${}^{3-}$ anions form two functionally different units working either as bridging units between three Ln${}^{3+}$ cations or as terminal ligands for Ln${}^{3+}$. [LnS${}_8$]${}^{13-}$ polyhedra form ${}_{\infty }^1${[LnS${}_{2/2}^v$S${}_{6/1}^t$]${}^{11-}$} 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 Ln${}^{3+}$ 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 Cs${}_3$La[AsS${}_4$]${}_3$ have confirmed the presence of [AsS${}_4$]${}^{3-}$ tetrahedra as structural building blocks.