Crystal structures of the double perovskites Ba{sub 2}Sr{sub 1-} {sub x} Ca {sub x} WO{sub 6}

• Published in: Journal of solid state chemistry 2007-05, Vol.180 (5)
• Main Authors: Fu, W.T., Akerboom, S., IJdo, D.J.W.
• Format: Article
• Language: English
• Subjects: BARIUM COMPOUNDS; CALCIUM COMPOUNDS; CUBIC LATTICES; INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; MONOCLINIC LATTICES; OXIDES; PEROVSKITES; SPACE GROUPS; STRONTIUM COMPOUNDS; TETRAGONAL LATTICES; TRIGONAL LATTICES; TUNGSTEN COMPOUNDS; X-RAY DIFFRACTION
• ISSN: 0022-4596; 1095-726X
• DOI: 10.1016/j.jssc.2007.03.008

Structures of the double perovskites Ba{sub 2}Sr{sub 1-} {sub x} Ca {sub x} WO{sub 6} have been studied by the profile analysis of X-ray diffraction data. The end members, Ba{sub 2}SrWO{sub 6} and Ba{sub 2}CaWO{sub 6}, have the space group I2/m (tilt system a {sup 0} b {sup -} b {sup -}) and Fm3-barm (tilt system a {sup 0} a {sup 0} a {sup 0}), respectively. By increasing the Ca concentration, the monoclinic structure transforms to the cubic one via the rhombohedral R3-bar phase (tilt system a {sup -} a {sup -} a {sup -}) instead of the tetragonal I4/m phase (tilt system a {sup 0} a {sup 0} c {sup -}). This observation supports the idea that the rhombohedral structure is favoured by increasing the covalency of the octahedral cations in Ba{sub 2} MM'O{sub 6}-type double perovskites, and disagrees with a recent proposal that the formation of the {pi}-bonding, e.g., d {sup 0}-ion, determines the tetragonal symmetry in preference to the rhombohedral one. - Graphical abstract: Enlarged sections showing the evolution of the basic (222) and (400) reflections in Ba{sub 2}Sr{sub 1-} {sub x} Ca {sub x} WO{sub 6}. Tick marks below are the positions of Bragg's reflections calculated using the space groups I2/m (x=0), R3-bar (x=0.25, 0.5 and 0.75) and Fm3-barm (x=1), respectively.