Abstract
The formula Ln5OF5Se4 (Ln = Nd and Sm) represents a new composition for lanthanoid(III) oxide fluoride selenides. Nd5OF5Se4 could be seen as aristotype of this system, crystallizing in the tetragonal crystal system with space group I4/mmm and the lattice parameters a = 404.53(2) pm and c = 3400.02(18) pm (c/a = 8.4, Z = 2). In contrast to the aristotype, Sm5OF5Se4 exhibtis a symmetry reduction and adopts space group Ccce with lattice parameters of a = 564.85(3) pm, b = 3391.79(18) pm, c = 564.83(3) pm (b/c ≈ b/a = 6.0, Z = 4). Both compounds can be described by a layered arrangement of sheets, which are formed by the coordination polyhedra surrounding the three crystallographically independent Ln3+ cations in eight- and ninefold coordination of anions. For reasons of electroneutrality one light-anion position has to be mixed-occupied with oxide (O2−) and fluoride (F−) anions for both compounds. Ln5OF5Se4, with edge-sharing [FLn4]11+ and [(O,F)Ln4]10.75+ tetrahedra linked to single and double layers separated by Se2− slabs, shows strong structural analogies to Er3OF3S2. In the case for Sm5OF5Se4 the absence of Sm2+ has been well supported by bond-valence calculations and XANES spectroscopy, while recorded DRS measurements were ambiguous owing to the red color of the substance.
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