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Following up the recently reported example La 3 F 2 Se 2 TaO 4 , colorless needle-shaped single crystals of the new compound La 2 FSeTaO 4  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. La 2 FSeTaO 4  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 Ta 5+  cations are surrounded by five oxygen atoms as square pyramids [TaO 5 ] 5– , which share  trans -oriented vertices ( d (Ta–O4) = 202 pm, 2×) to form straight∞   1TaO   1   ,   2   ,   33   /   1tO   42   /   2v3   −${}_{\infty }^{1}\left\{{\left[\text{Ta}{\left.\text{O}1,2,3\right.}_{3/1}^{t}{\left.\text{O}4\right.}_{2/2}^{v}\right]}^{3-}\right\}$chains propagating along [010] with isotactically aligned tips ( d (Ta–O1) = 183 pm). Embedded into a matrix of lanthanum fluoride selenide according to∞   3La   2FSe3   +${}_{\infty }^{3}\left\{{\left[{\text{La}}_{2}\text{FSe}\right]}^{3+}\right\}$( d (La–F) = 236–244 pm,  d (La–Se) = 312–318 pm), which consists of two different La 3+  cations as well as singular F −  and Se 2−  anions, via La–O contacts ( d (La–O) = 247–274 pm), the coordination number of the La 3+  cation towards the anions F − , O 2−  and Se 2– amounts to nine with tricapped trigonal prisms or capped square antiprisms as coordination polyhedra models. The tantalum cation has a distant contact with a Se 2−  anion ( d (Ta⋯Se) = 317 pm), not efficiently counting to its first coordination sphere, but completing it to a  quasi- octahedron.

La 2 FSeTaO 4 : lanthanum fluoride selenide oxotantalate or lanthanum fluoride oxoselenotantalate?