The Structural Dimorphism of Lanthanum Oxide Fluoride Selenide La₂OF₂Se

• Main Authors: Constantin Buyer, Hagen Grossholz, Thomas Schleid
• Format: Article
• Language: English
• Published: MDPI AG 2019-08-01
• Series: Crystals
• Subjects: lanthanum oxide fluoride selenides; polymorphism; X-ray diffraction; crystal structure; rare-earth metal compounds
• Online Access: https://www.mdpi.com/2073-4352/9/9/435

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 &#176;C. Needle-shaped crystals of BLa₂OF₂Se 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 &#176;C. The Atype form of La₂OF₂Se 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>6₃/<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-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&#8722; and [LaF₇Se₃]^10&#8722; polyhedra typical for the ternaries. B-La₂OF₂Se appears to be structurally very similar to La₆O₂F₈Se₃ in displaying [LaO₃FSe₄]^12&#8722; and [LaOF₆Se₂]^9&#8722; polyhedra. With 6.039 versus 6.036 g/cm&#179; the B-type form of La₂OF₂Se is slightly denser than the A-type variant.