Ein neues Selten‐Erd‐Metall(III)‐Fluorid‐Oxoselenat(IV): YF[SeO3]

A New Rare‐Earth Metal(III) Fluoride Oxoselenate(IV): YF[SeO₃] Just two representatives of the rare‐earth metal(III) fluoride oxoselenates(IV) with the formula type MF[SeO₃] (M = La and Lu) exist so far, whereas for the intermediate lanthanoids only M₃F[SeO₃]₄‐type compounds (M = Gd and Dy) were accessible. Because of the similar radius of Y³⁺ to the radii of the heavier lanthanoid cations, a missing link within the MF[SeO₃] series could be synthesized now with the example of yttrium(III) fluoride oxoselenate(IV). Contrary to LuF[SeO₃] with its triclinic structure, YF[SeO₃] crystallizes monoclinically in space group P2₁/c (no. 14, a = 657.65(7), b = 689.71(7), c = 717.28(7) pm, β = 99.036(5)° and Z = 4). A single Y³⁺ cation occupying the general site 4e is surrounded by six oxide and two fluoride anions forming [YO₆F₂]^11? polyhedra (d(Y–O) = 228–243 plus 263 pm, d(Y–F) = 219–220 pm). These are linked via common O···O edges to chains running along [010] and adjacent chains get tied to each other by sharing common O3···O3 and O3···F edges which results in sheets parallel to (100). The Se⁴⁺ cations connect these sheets as ψ¹‐tetrahedral [SeO₃]^2? anions (d(Se–O) = 168–174 pm) for charge balance via all oxygen atoms. Despite the different coordination numbers of seven and eight for the rare‐earth metal(III) cations the structures of LuF[SeO₃] and YF[SeO₃] appear quite similar. The chains containing pentagonal bipyramids [LuO₅F₂]^9? are connected to layers running parallel to the (100) plane again. In fact it is only necessary to shorten the partial structure of the straight chains along [001] to achieve the angular chains in YF[SeO₃]. As a result of this shortening one oxide anion at a time moves into the coordination sphere of a neighboring Y³⁺ cation and therefore adds up the coordination number for Y³⁺ to eight. For the synthesis of YF[SeO₃] yttrium sesquioxide (Y₂O₃), yttrium trifluoride (YF₃) and selenium dioxide (SeO₂) in a molar ratio of 1 : 1 : 3 with CsBr as fluxing agent were reacted within five days at 750 °C in evacuated graphitized silica ampoules.