Über hexagonale Perowskite mit Kationenfehlstellen. XVII Strukturbestimmung an Ba9Nb6W□2O27 - der ersten Stapel-variante eines rhomboedrischen 27 L-Typs

On Hexagonal Perovskites with Cationic Vacancies. XVII. Structure Determination on Ba₉Nb₆W□₂O₂₇ – the First Stacking Polytype of a Rhombohedral 27 L-Type The hexagonal stacking polytype of rhombohedral 27 L -type, Ba₉Nb₆W□₂O₂₇, crystallizes in the space group R3 m with the sequence (4)1(3)1 ? (hhccchhcc)₃ and three formula units for the trigonal setting. The refined, intensity related, R'-value is 9.7percnt;. The octahedral net consists of blocks of three face connected octahedra which are linked to each other alternately through one or two octahedra connected exclusively by common vertices. The cationic vacancies are located in the centers of the groups of three octahedra. With this distribution direct contact between occupied face-sharing octahedra is avoided. The niobium and tungsten atoms are distributed statistically between the remaining octahedral holes. In the blocks of three octahedra they are displaced by ≈ 0.29 Å from their ideal positions in the direction of the central void. The Ba atoms neighbouring a vacancy (all in hexagonal packed BaO₃ sheets) are dislocated in the direction of the void, while the cubic packed BaO₃ sheets maintain nearly regular form.