Uber hexagonale Perowskite mit Kationenfehlstellen. VI. Strukturbestimmung an Ba8Re2W3□3O24 – der ersten hexagonalen Perowskitstapelvariante vom 24 L-Typ

On Hexagonal Perovskites with Cationic Vacancies. VI. Structure Determination on Ba₈Re₂W₃□₃O₂₄ – the First Hexagonal Stacking Polytype with 24 Layers For the hexagonal stacking polytype of rhomboedral 24 L type, Ba₈Re₂W₃□₃O₂₄, eight sequences are possible: (6)(2); (4)1(2)1; (5)1(1)1; (5)(3); (3)11(1)11; (1)21(1)12 (all space group R3m) and 4211; 3221 (all space group R3m). By vibrational spectroscopic investigations the sequences (6)(2); (5)(3) and 4211 could be eliminated. By means of intensity calculations the sequence (1)21(1)12 or chhhhchh respectively can be selected out of the remaining five. The structure contains groups of three and five face-connected octahedra which are linked with another by common corners. In the ideal structure, vacant lattice sites are found in the centres of the second and fourth octahedra of the block of five face-sharing octahedra and in the central octahedron of the block of three face-sharing octahedra. The remaining positions are occupied by the Re and W atoms. According to this distribution an direct contact of occupied face-sharing octahedra is absent. In the real structure, lattice defects are introduced by partial redistribution of atoms between occupied and vacant lattice positions in the blocks of three and five face-sharing octahedra. The occupation factor of one vacancy is about 8%. The refined, intensity related R′ value is 15.2%.