La2FSeTaO4: lanthanum fluoride selenide oxotantalate or lanthanum fluoride oxoselenotantalate?

Following up the recently reported example La3F2Se2TaO4, colorless needle-shaped single crystals of the new compound La2FSeTaO4 were obtained as an unexpected by-product from solid-state reactions between lanthanum metal, lanthanum trifluoride, lanthanum sesquioxide and selenium in a sodium-chloride flux designed to yield lanthanum oxide fluoride selenides. Silicajacketed tantalum capsules served as reaction containers at 850 degrees C, but an undesired activation of the metal tubes led to the formation of a new oxotantalate. La2FSeTaO4 crystallizes orthorhombically in the space group Pnma with the lattice parameters a = 1146.51(6), b = 395.16(2) and c = 1281.32(7) pm for Z = 4. The crystallographically unique Ta5+ cations are surrounded by five oxygen atoms as square pyramids [TaO5](5-), which share trans-oriented vertices (d(Ta-O4) = 202 pm, 2x) to form straight (1)(infinity){Ta(O1, 2, 3)(3/1)(t) (O4)(2/2)(v)](3-)} chains propagating along [010] with isotactically aligned tips (d(Ta-O1) = 183 pm). Embedded into a matrix of lanthanum fluoride selenide according to (3)(infinity){[La2FSe](3+)} (d(La-F) = 236-244 pm, d(La-Se) = 312-318 pm), which consists of two different La3+ cations as well as singular F- and Se2- anions, via La-O contacts (d(La-O) = 247-274 pm), the coordination number of the La3+ cation towards the anions F-, O2- and Se2- amounts to nine with tricapped trigonal prisms or capped square antiprisms as coordination polyhedra models. The tantalum cation has a distant contact with a Se2- anion (d(Ta.Se) = 317 pm), not efficiently counting to its first coordination sphere, but completing it to a quasi-octahedron.