Crystal structure of Ba9La2W4O24 and Sr9Gd2W4O24: A new B-site vacancy ordered 4 x 4 x 4 cubic perovskite

The crystal structure of Ba9La2W4O24 and the high-temperature modification Sr9Gd2W4O24 has been investigated by the Rietveld method using X-ray powder diffraction data. They crystalize in a 4 x 4 x 4 cubic superstructure of the simple cubic perovskite ABO(3) with the space group Fm (3) over bar and lattice parameter a approximate to 4a(p), where a(p) denotes the lattice constant of the primitive cubic perovskite. The structure consists of the ordered B-site vacancies (square) and the structure formula can be written as Ba-2(Ba1/4La1/2)square 1/4WO6 and (Sr/Gd)(2)((Sr/Gd)(3/4))square 1/4WO6 due to a partial disorder between one of the Sr at A-site and the Gd at B-site. The three crystallographically different W and the randomly distributed Ba/La or (Sr/Gd) ions occupy the B-sites. Two W ions are coordinated with 6 oxygens and arranged as octahedra, but the third one is surrounded by 12 partially occupied (50%) oxygens, which can be viewed as a combination of four tetrahedra. The coordination geometry of the larger B-cations (Ba/La and Sr/Gd) is not octahedral; they coordinate with seven oxygens instead. In addition, the Ba/LaO7 or (Sr/GdO7) polyhedron shares some oxygen atoms with four neighbours. The B-site vacancies form a primitive cube with a approximate to 2a(p), and are surrounded by six equal but differently orientated WO6 octahedra being caused by a rotation about a twofold axis. (C) 2016 Elsevier Inc. All rights reserved.