About lanthanoid fluoride selenide oxoselenotantalates with the composition Ln3F2Se2TaO4 (Ln = La - Nd)

Abstract

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 Ln3F2Se2TaO4 (Ln = La - Nd) were obtained. They crystallize in the orthorhombic space group Pnma analogous to La3F2Se2NbO4 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 Ln3+ cations are all surrounded by nine anions (O2-, F- and Se2-) each. Tantalum resides in an octahedral chalcogen coordination by forming trans-vertex oxygen-connected [TaO5Se]7- polyhedra, which build up chains 1∞{[TaOV2/2Ot3/1Set1/1]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 Se2- anions have five or six cationic neighbors.