Über geordnete Perowskite mit Kationenfehlstellen. VIII. Strukturuntersuchungen an Ba2Ba7/8□1/8UO57/8□1/8

On ordered perovskites with cationic vacancies. VIII. Structure investigations on Ba₂Ba_7/8□_1/8UO_57/8□_1/8 The reddish brown Ba₂Ba_7/8□_1/8UO_57/8□_1/8 belongs to the group of oxygen perovskites with an ordered distribution of cationic vacancies. It crystallized tetragonally (a = 12.62₄ Å; c = 17.53₄ Å) with 16 formula units in the unit cell: Ba₃₂Ba₁₄□₂U₁₆O₉₄□₂. For the space group I4/mmm intensity calculations on powder data gave a refined, intensity related R′ value of 13.4%. The octahedrally coordinated barium an uranium atoms are 1:1 ordered; both cationic vacancies are located in the barium sublattice and form a body centered arrangement. For the 94 oxygen atoms and the two oxygen vacancies a statistical distribution was chosen. In the lattice all cations neighbouring the cationic vacancies are dislocated: The corresponding barium atoms in the close packed sheets move by ～0.55 Å in direction of the holes, on the contary the uranium atoms concerned are shifted away by ～0.17 Å and ～0.26 Å respectively.