Zur Kenntnis ternärer Oxoarsenate(III) dreiwertiger Lanthanoide: Synthese und Charakterisierung von LnAsO3- und Ln 2As4O9-Vertretern mit Ln = La und Ce sowie Ln = Pr, Nd, Sm–Gd

Abstract The title compounds have been prepared from the elemental lanthanoids and arsenic sesquioxide in 1:1 and 2:3 molar ratios in the presence of a cesium-halide flux at temperatures near 1000 °C generating elemental arsenic as the by-product. With the preparation of B -type La[AsO 3 ] crystallizing in the monoclinic space group P 2 1 / c with the lattice parameters a = 904.03(8), b = 789.96(7), c = 837.49(7) pm and β = 103.354(3)°, both A - and B -type Ln [AsO 3 ] representatives are now available for lanthanum and cerium. The crystal structure consists of two crystallographically distinct Ln 3+ cations with eight- or ninefold coordination by O 2− anions. The Ln 3+ cations are arranged in layers of ( Ln 1) 3+ and corrugated bilayers of ( Ln 2) 3+ cations in the (100) plane. The two different As 3+ cations appear in [AsO 3 ] 3− units, which differ mainly in their environment of Ln 3+ cations. The successful synthesis of the compounds Ln 2 As 4 O 9 ( Ln = Pr, Nd, Eu and Gd) reported here on the one hand closes the structural gap between cerium and neodymium, and on the other hand also extends the spectrum of ternary oxoarsenates(III) of lanthanoids to gadolinium, with the neodymium compound for the first time at standard temperature. The crystals of the Ln 2 As 4 O 9 compounds are triclinic (space group: P 1 ‾ $P\bar{1}$ ) with lattice parameters nicely reflecting the lanthanoid contraction from a = 690.61(5), b = 768.95(6), c = 959.72(8) pm, α = 96.927(3), β = 103.812(3), γ = 104.441(3)° for Pr 2 As 4 O 9 to a = 676.12(5), b = 751.58(6), c = 951.57(8) pm, α = 96.462(3), β = 103.678(3), γ = 104.409(3)° for Gd 2 As 4 O 9 with two crystallographically independent lanthanoid(III) positions. The oxidic environments on these sites show the shape of square antiprisms with significant distortions, but ( Ln 2) 3+ has a ninth oxygen contact in a capped antiprism. The linkage of the polyhedra creates layers in the ab plane, which are separated by the complex anions of arsenic. The oxoarsenate(III) units [AsO 3 ] 3− are linked by sharing oxygen corners to form both dinuclear [As 2 O 5 ] 4− and cyclic tetranuclear [As 4 O 8 ] 4− complex anions, which is why the compounds can also be represented with their structured molecular formula Ln 4 [As 2 O 5 ] 2 [As 4 O 8 ]. The free electron pairs of the arsenic(III) cations point into channels, which are built up in the crystal structure along [010].