Synthesis and characterization of La₃NbSe₂O₄F₂

Crystals of the unusual oxyfluoroselenide La₃NbSe₂O₄F₂ were obtained during the exploration of the quaternary La/Nb/Cu/Se system. Oxygen was extracted from the silica tube, while fluorine was present as a minor impurity in the La powder. The compound crystallizes in space group D¹⁶_2h-Pnma of the orthorhombic system with four formula units in a cell with dimensions: a = 11.290(4), b = 4.001(1), and c = 18.062(4) Å (T = 113 K). The structure has been determined by single-crystal X-ray methods. The presence of F in the crystals was confirmed by windowless EDAX measurements. The two F sites were distinguished from the four O sites from a combination of the X-ray refinement and a bond-valence parameter calculation made with the program EUTAX. In the structure the Nb atom is octahedrally coordinated while each of the three independent La atoms is in a tricapped trigonal prism. The Nb atom is bound to one Se atom and five O atoms while the three La sites are coordinated by various combinations of Se, O, and F atoms. The NbO₅Se octahedra corner share and the LaSe_xO_yF_z tricapped trigonal prisms face share in the b direction.