Nd5OF5Se4 and Sm5OF5Se4: New layered oxide fluoride selenides of the lanthanoids

The formula Ln(5)OF(5)Se(4) (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 approximate to 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 Ln(3+) 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. Ln(5)OF(5)Se(4), with edge-sharing [FLn(4)](11)+ and [(O,F)Ln(4)](10.75+) tetrahedra linked to single and double layers separated by Se-2 -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.

Automated summary

The study presents a new composition for lanthanoid(III) oxide fluoride selenides with different crystal structures and space groups. Both compounds exhibit a layered arrangement with Ln(3+) cations surrounded by coordinating anions, and a design involving mixed occupancy of oxide and fluoride anions for electroneutrality.