Abstract
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.
Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
