| Summary: | The new colorless lanthanum oxide fluoride selenide La<sub>2</sub>OF<sub>2</sub>Se could be synthesized via solid-state reactions in two different structure types. Lamellar crystals of A-La<sub>2</sub>OF<sub>2</sub>Se were obtained from mixtures of La, LaF<sub>3</sub>, La<sub>2</sub>O<sub>3</sub> 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 °C. Needle-shaped crystals of BLa<sub>2</sub>OF<sub>2</sub>Se emerged from reactions of the same educt mixtures in molar ratios of 6:4:4:9 scheduled to produce La<sub>6</sub>O<sub>4</sub>F<sub>4</sub>Se<sub>3</sub> with CsI as flux for four days in niobium ampoules at 700 °C. The Atype form of La<sub>2</sub>OF<sub>2</sub>Se crystallizes in the trigonal space group <i>R</i>3m with <i>a</i> = 418.13(3) and <i>c</i> = 4478.2(4) pm for <i>Z</i> = 6, whereas the Btype form is hexagonal (space group: <i>P</i>6<sub>3</sub>/<i>m</i>) with <i>a</i> = 1396.82(9) and <i>c</i> = 401.08(3) pm for <i>Z</i> = 6. The crystal structure of A-La<sub>2</sub>OF<sub>2</sub>Se shows a close relationship to the fluoride-free La<sub>2</sub>O<sub>2</sub>Se and the oxygen-free La<sub>2</sub>F<sub>4</sub>Se. It can even be discussed as 1:1 intergrowth variety, since it contains the [LaO<sub>4</sub>Se<sub>3</sub>]<sup>11−</sup> and [LaF<sub>7</sub>Se<sub>3</sub>]<sup>10−</sup> polyhedra typical for the ternaries. B-La<sub>2</sub>OF<sub>2</sub>Se appears to be structurally very similar to La<sub>6</sub>O<sub>2</sub>F<sub>8</sub>Se<sub>3</sub> in displaying [LaO<sub>3</sub>FSe<sub>4</sub>]<sup>12−</sup> and [LaOF<sub>6</sub>Se<sub>2</sub>]<sup>9−</sup> polyhedra. With 6.039 versus 6.036 g/cm³ the B-type form of La<sub>2</sub>OF<sub>2</sub>Se is slightly denser than the A-type variant.
|
|---|
