La2FSeTaO4: lanthanum fluoride selenide oxotantalate or lanthanum fluoride oxoselenotantalate?

Abstract

Abstract 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. Silica-jacketed tantalum capsules served as reaction containers at 850 °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, 2×) to form straight ∞ 1 Ta O 1 , 2 , 3 3 / 1 t O 4 2 / 2 v 3 − ${}_{\infty }^{1}\left\{{\left[\text{Ta}{\left.\text{O}1,2,3\right.}_{3/1}^{t}{\left.\text{O}4\right.}_{2/2}^{v}\right]}^{3-}\right\}$ chains propagating along [010] with isotactically aligned tips (d(Ta–O1) = 183 pm). Embedded into a matrix of lanthanum fluoride selenide according to ∞ 3 La 2 FSe 3 + ${}_{\infty }^{3}\left\{{\left[{\text{La}}_{2}\text{FSe}\right]}^{3+}\right\}$ (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.