LuF[SeO3] und LuCl[SeO3]: Zwei nicht‐isotype Halogenid‐Oxoselenate(IV) des Lutetiums

LuF[SeO₃] and LuCl[SeO₃]: Two Non‐Isotypic Halide Oxoselenates(IV) of Lutetium Despite the formal similarity of LuF[SeO₃] and LuCl[SeO₃] both compounds show significant structural differences due to the different positions of the halide anions (X⁻) within the pentagonal bipyramids [LuO₅X₂]⁹⁻. However, both oxoselenates(IV) have these pentagonal bipyramids as basic modules in common that are connected via O²⁻ edges to chains. LuCl[SeO₃] crystallizes orthorhombically in space group Pnma (no. 62; a = 714.63(7), b = 681.76(7) and c = 864.05(9) pm; Z = 4). The structure is isotypic to that one recently presented for ErCl[SeO₃]. With a single Cl⁻ anion in each an apical and an equatorial position, the chains have to be inclined with an angle of about 54° relative to each other to get connected alternately by common Cl⁻ corners and bridging [SeO₃]²⁻ pyramids. In contrast to that, LuF[SeO₃] which crystallizes triclinically in space group (no. 2; a = 644.85(6), b = 684.41(7), c = 427.98(4) pm, α = 94.063(5), β = 96.484(5) and γ = 91.895(5)°; Z = 2) takes a structural motif already known from CsTmCl₂[SeO₃]. Owing to the apical position of both halide anions it is now possible to connect the chains directly via discrete Ψ¹‐tetrahedral [SeO₃]²⁻ groups to layers. The same layers are present in LuF[SeO₃] and without the formal alkali‐metal halide unit (CsCl) of the CsTmCl₂[SeO₃]‐type compounds, the layers can also be connected directly by common F⁻ corners to a three‐dimensional array. Torch‐sealed evacuated silica ampoules were used for the synthesis of both lutetium(III) halide oxoselenates(IV). For LuF[SeO₃] these vessels have been graphitized before to prevent them from oxosilicate‐producing side‐reactions with the applied fluoride. The synthesis of LuCl[SeO₃] required Lu₂O₃ and SeO₂ in a molar ratio of 1 : 6 with a surplus of an eutectic RbCl/LiCl mixture as fluxing agent and an annealing period of five weeks at a temperature of 500 °C, whereas Lu₂O₃, LuF₃ and SeO₂ (in a molar ratio of 1 : 1 : 3) with CsBr as flux were converted to LuF[SeO₃] at 750 °C within six days.