Crystal structures of the double perovskites Ba2Sr1−xCaxWO6

Structures of the double perovskites Ba2Sr1−xCaxWO6 have been studied by the profile analysis of X-ray diffraction data. The end members, Ba2SrWO6 and Ba2CaWO6, have the space group I2/m (tilt system a0b−b−) and Fm3¯m (tilt system a0a0a0), respectively. By increasing the Ca concentration, the monoclinic structure transforms to the cubic one via the rhombohedral R3¯ phase (tilt system a−a−a−) instead of the tetragonal I4/m phase (tilt system a0a0c−). This observation supports the idea that the rhombohedral structure is favoured by increasing the covalency of the octahedral cations in Ba2MM′O6-type double perovskites, and disagrees with a recent proposal that the formation of the π-bonding, e.g., d0-ion, determines the tetragonal symmetry in preference to the rhombohedral one.