The Structural Dimorphism of Lanthanum Oxide Fluoride Selenide La2OF2Se

The new colorless lanthanum oxide fluoride selenide La2OF2Se could be synthesized via solid-state reactions in two different structure types. Lamellar crystals of A-La2OF2Se were obtained from mixtures of La, LaF3, La2O3 and Se in molar ratios of 2:2:1:3 with NaCl as flux for seven days in silica-protected sealed tantalum capsules at 850 degrees C. Needle-shaped crystals of B-La2OF2Se emerged from reactions of the same educt mixtures in molar ratios of 6:4:4:9 scheduled to produce La6O4F4Se3 with CsI as flux for four days in niobium ampoules at 700 degrees C. The A-type form of La2OF2Se crystallizes in the trigonal space group R3 over bar m with a = 418.13(3) and c = 4478.2(4) pm for Z = 6, whereas the B-type form is hexagonal (space group: P6(3)/m) with a = 1396.82(9) and c = 401.08(3) pm for Z = 6. The crystal structure of A-La2OF2Se shows a close relationship to the fluoride-free La2O2Se and the oxygen-free La2F4Se. It can even be discussed as 1:1 intergrowth variety, since it contains the [LaO4Se3](11-) and [LaF7Se3](10-) polyhedra typical for the ternaries. B-La2OF2Se appears to be structurally very similar to La6O2F8Se3 in displaying [LaO3FSe4](12-) and [LaOF6Se2](9-) polyhedra. With 6.039 versus 6.036 g/cm(3) the B-type form of La2OF2Se is slightly denser than the A-type variant.