The isotypic series of tetragonal lanthanoid(III) fluoride oxoarsenates(III) Ln₅F₃[AsO₃]₄ (Ln = Eu–Lu).

The new fluoride-containing lanthanoid(III) oxoarsenates(III) with the composition Ln5F3[AsO3]4 (Ln = Eu–Lu) could be obtained through partial metallothermic reduction from mixtures of the corresponding trifluorides (LnF3) and the metals (Ln) with As2O3 upon heating in fused silica ampoules. They all crystallize tetragonally in the space group P4/ncc with lattice parameters ranging from a = 1164.71(8) pm and c = 1097.95(7) pm for Eu5F3[AsO4]4 down to a = 1125.72(8) pm and c = 1061.16(7) pm for Lu5F3[AsO3]4 for Z = 4 as consequence of the lanthanoid contraction. The crystal structure exhibits three positions for the trications occupied by one As3+ and two Ln3+ species. The lanthanoids occur with two different coordination polyhedra consisting of a fluoride-capped oxygen cube [(Ln1)O8F]14− and a bicapped trigonal prism [(Ln2)O6F2]11− of F− and O2− anions. A surprisingly diverse Ln3+ cation coordination for the F− anions is noteworthy, as (F1)− has contact to five, but (F2)− to only two of them. The crystallographically unique As3+ cation is coordinated in a discrete ψ1-tetrahedron [AsO3]3− by three oxygen atoms. The lone-pairs of electrons at the As3+ centers point into empty channels along [001], flanked by four columns of condensed [(Ln1)O8F]14− polyhedra. As rather short oxygen-fluorine distances are present within this structural feature, the absence of (OH)− units apt to replace F− needed to be verified by single-crystal Raman spectroscopy. The composition Ln5F3[AsO3] was also confirmed by electron-beam microprobe (EDXS) measurements and the phase purity of selected samples could be proven by powder X-ray diffractometry (PXRD). [ABSTRACT FROM AUTHOR]