The First Members of the Monoclinic Lanthanoid(III) Fluoride Oxoarsenate(III) Series <i>Ln</i>₂F₂[As₂O₅] (<i>Ln</i>=La – Nd and Sm)

Abstract In synthesis experiments for the preparation of Ln 5 F 3 [AsO 3 ] 4 representatives in evacuated glassy silica ampoules at temperatures of maximum 825 °C, surprisingly compounds with the composition Ln 2 F 2 [As 2 O 5 ] could be obtained and subsequently also reproduced on target. The new Ln 2 F 2 [As 2 O 5 ] representatives with Ln = La – Nd and Sm crystallize needle‐shaped in the monoclinic space group P 2 1 / c with lattice parameters ranging from a = 793.24(5) pm, b = 1346.39(9) pm, c = 1359.41(9) pm and β = 106.932(3)° for La 2 F 2 [As 2 O 5 ] to a = 769.11(5) pm, b = 1307.50(9) pm, c = 1328.74(9) pm and β = 106.819(3)° for Sm 2 F 2 [As 2 O 5 ] with Z = 8. Their crystal structure can be broken down into two parts consisting of infinite [ Ln 2 F 2 ] 4+ and discrete [As 2 O 5 ] 4− units. Three distinct kinds of fluoride anions are trigonally planar surrounded by lanthanoid cations and another one carries even a fourth of them in their coordination sphere with a secondary contact resulting in a distorted tetrahedron. These [F Ln 3 ] 8+ and [F Ln 3+1 ] 11+ building blocks share common corners and edges creating strands {[F 2 Ln 2 ] 4+ }, which propagate along [100]. The four different As 3+ lone‐pair cations form ψ 1 ‐tetrahedra [AsO 3 ] 3− with three oxygen atoms and are conversurally linked pairwisely creating dinuclear pyroanionic [As 2 O 5 ] 4− units. These [As 2 O 5 ] 4− groups arrange themselves alternatingly along [100] with their lone pairs pointing into central empty channels. The {[F 2 Ln 2 ] 4+ } strands are linked to the [As 2 O 5 ] 4− units via four distinguishable Ln 3+ cations providing them coordination numbers of eight and nine. Raman spectroscopy confirmed that no (OH) − groups resulting from the synthesis were present apt to replace part of the F − and the stoichiometry Ln 2 F 2 [As 2 O 5 ] was verified using WDXS elemental analyses.