The rare earth metal(III) chloride oxidoarsenates(III) with the composition RE5Cl3[AsO3]4 (RE = La–Nd, Sm) could be synthesized via solid-state methods through the reaction of arsenic sesquioxide (As2O3) with the corresponding rare earth metal compounds (La2O3, CeO2 + metallic Ce, Pr6O11, Nd2O3 or metallic Sm) using several chloride-containing fluxing agents in evacuated silica glass ampoules. The compounds build up non-isotypic crystal structures in the monoclinic space groups C2/c for RE = La–Pr, and P2/c for RE = Nd and Sm. All rare earth metal(III) cations exhibit coordination numbers of eight. While (RE1)3+ and (RE2)3+ are only surrounded by oxygen atoms in the form of distorted square antiprisms or prisms, (RE3)3+ is coordinated square antiprismatically by four oxygen atoms and four chloride anions. Although the coordination polyhedra in both structures differ only marginally, their connection patterns show more pronounced differences. This regards especially the (RE)3+ cations and results from different site symmetries of the (Cl1)− anions. All As3+ lone-pair cations are coordinated by three oxygen atoms to form ψ1-tetrahedral [AsO3]3− complex anions with their non-binding (lone) electron pairs pointing into empty channels along [010].

Pale yellow single crystals of the composition Ln3X2[As2O5][AsO3] (Ln = Tm for X = Br and Ln = Sm for X = Cl) were obtained via solid‐state reactions in the systems Ln2O3/As2O3 from sealed silica ampoules using different halides as fluxing agents. Sm3Cl2[As2O5][AsO3] and Tm3Br2[As2O5][AsO3] crystallize isotypically in the triclinic space group P1 with Z = 2 and cell parameters of a = 543.51(4) pm, b = 837.24(6) pm, c = 1113.45(8) pm, α = 90.084(2)°, β = 94.532(2)°, γ = 90.487(2)° for the samarium and a = 534.96(4) pm, b = 869.26(6) pm, c = 1081.84(8) pm, α = 90.723(2)°, β = 94.792(2)° γ = 90.119(2)° for the thulium compound. The isotypic crystal structure of both representatives exhibits three crystallographically different Ln3+ cations, each with a coordination number of eight. (Ln1)3+ and (Ln2)3+ are only coordinated by three oxygen atoms, whereas (Ln3)3+ shows additional contacts to halide anions in forming square [LnO4X4]9– antiprisms. All As3+ cations are surrounded by three oxygen atoms in the shape of isolated [AsO3]3– ψ1‐tetrahedra. They occur either isolated or condensed as pyroanionic [As2O5]4– units with a bridging oxygen atom. In both anions, non‐binding lone‐pair electrons are present at the As3+ cations with a pronounced stereochemically active function.