About lanthanoid fluoride selenide oxoselenotantalates with the composition Ln${}_3$F${}_2$Se${}_2$TaO${}_4$ (Ln = La - Nd)

After solid-state reactions of the light lanthanoid metals, their oxides and fluorides as well as selenium in sealed tantalum ampoules with sodium chloride as a fluxing agent at 850 °C for 8 days needle-shaped single crystals of Ln${}_3$F${}_2$Se${}_2$TaO${}_4$ (Ln = La - Nd) were obtained. They crystallize in the orthorhombic space group Pnma analogous to La${}_3$F${}_2$Se${}_2$NbO${}_4$ with a = 1133-1120 pm, b = 400-393 pm and c = 1812-1778 pm (Ln = La - Nd) for Z = 4 as the first known quinary lanthanoid(III) oxoselenotantalates(V) with fluoride and selenide anions. The three crystallographically different Ln${}^{3+}$ cations are all surrounded by nine anions (O${}^{2-}$, F${}^{-}$ and Se${}^{2-}$) each. Tantalum resides in an octahedral chalcogen coordination by forming trans-vertex oxygen-connected [TaO${}_5$Se]${}^{7-}$ polyhedra, which build up chains ${}_{\infty }^1${[TaO${}_{2/2}^V$O${}_{3/1}^t$Se${}_{1/1}^t$]${}^{5-}$ along [010]. The sites of the four crystallographically different oxygen atoms and the two distinct fluoride anions were established by bond-valence calculations. One fluorine and three oxygen atoms are surrounded tetrahedrally by cations, while another fluoride and oxide anion exhibit just triangular non-planar coordination spheres. The two independent Se${}^{2-}$ anions have five or six cationic neighbors. (©2020 The Authors published by Wiley-VCH GmbH, Weinheim)