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

• Ba9La2W4O24 and Sr9Gd2W4O24 are a new-type of B-site vacancy ordered perovskites.
• The structure consists of a 4×4x4 superstructure of the cubic perovskite.
• One W is surrounded by 12 partially occupied (50%) oxygens.
• the larger B-cation polyhedron shares oxygens with four neighbours.

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 4×4×4 cubic superstructure of the simple cubic perovskite ABO3 with the space group Fm3¯ and lattice parameter a≈4ap, where ap denotes the lattice constant of the primitive cubic perovskite. The structure consists of the ordered B-site vacancies (☐) and the structure formula can be written as Ba2(Ba1/4La1/2)☐1/4WO6 and (Sr/Gd)2((Sr/Gd)3/4)☐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≈2ap, and are surrounded by six equal but differently orientated WO6 octahedra being caused by a rotation about a twofold axis.

Graphical Abstract(a) Schematic representation of the crystal structure of Ba9La2W4O24 showing the connection of B-cation polyhedra. Note that oxygens with partial occupancy (O1) are shaded with two colours The A-site Ba atoms are shaded with grey. (b) and (c) show the projections of thin slices at z≈0 and z≈1/4, respectively. The B-cation vacancies at z=1/4 are shown as black dots. Slices at z≈1/2 and z≈3/4 can be obtained by shifting the previous ones by 1/2a.Figure optionsDownload as PowerPoint slide