The isotypic series of tetragonal lanthanoid(III) fluoride oxoarsenates(III) Ln ₅F₃[AsO₃]₄ (Ln = Eu–Lu)

The new fluoride-containing lanthanoid(III) oxoarsenates(III) with the composition Ln 5 F 3 [AsO 3 ] 4 ( Ln = Eu–Lu) could be obtained through partial metallothermic reduction from mixtures of the corresponding trifluorides ( Ln F 3 ) and the metals ( Ln ) with As 2 O 3 upon heating in fused silica ampoules. They all crystallize tetragonally in the space group P 4/ ncc with lattice parameters ranging from a = 1164.71(8) pm and c = 1097.95(7) pm for Eu 5 F 3 [AsO 4 ] 4 down to a = 1125.72(8) pm and c = 1061.16(7) pm for Lu 5 F 3 [AsO 3 ] 4 for Z = 4 as consequence of the lanthanoid contraction. The crystal structure exhibits three positions for the trications occupied by one As 3+ and two Ln 3+ species. The lanthanoids occur with two different coordination polyhedra consisting of a fluoride-capped oxygen cube [( Ln 1)O 8 F] 14− and a bicapped trigonal prism [( Ln 2)O 6 F 2 ] 11− of F − and O 2− anions. A surprisingly diverse Ln 3+ cation coordination for the F − anions is noteworthy, as (F1) − has contact to five, but (F2) − to only two of them. The crystallographically unique As 3+ cation is coordinated in a discrete ψ 1 -tetrahedron [AsO 3 ] 3− by three oxygen atoms. The lone-pairs of electrons at the As 3+ centers point into empty channels along [001], flanked by four columns of condensed [( Ln 1)O 8 F] 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 Ln 5 F 3 [AsO 3 ] 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).