Crystal structures of the double perovskites Ba 2Sr 1−xCa xWO 6

Abstract

Structures of the double perovskites Ba 2Sr 1− x Ca x WO 6 have been studied by the profile analysis of X-ray diffraction data. The end members, Ba 2SrWO 6 and Ba 2CaWO 6, have the space group I2/ m (tilt system a 0 b − b −) and Fm3¯ m (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 R3¯ 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 Ba 2 MM′O 6-type double perovskites, and disagrees with a recent proposal that the formation of the π-bonding, e.g., d 0-ion, determines the tetragonal symmetry in preference to the rhombohedral one.