Crystal structures of the double perovskites Ba2Sr1-xCaxWO6

Structures of the double perovskites Ba(2)Srl-,Ca,WO6 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 a(0)b(-)b(-)) and Fm (3) over barm (tilt system a(0)a(0)a(0)), respectively. By increasing the Ca concentration, the monoclinic structure transforms to the cubic one via the rhombohedral R (3) over bar phase (tilt system a(-)a(-)a(-)) instead of the tetragonal I4/m phase (tilt system a(0)a(0)c(-)). This observation supports the idea that the rhombohedral structure is favoured by increasing the covalency of the octahedral cations in Ba2MMO6-type double perovskites, and disagrees with a recent proposal that the formation of the pi-bonding, e.g., d(0)-ion, determines the tetragonal symmetry in preference to the rhombohedral one. (C) 2007 Elsevier Inc. All rights reserved.