The intermediate tilt systems in the double perovskites Ba1−xSrxCaWO6 and Ba2−xSrxYSbO6

To understand the roles of the A- and B-cation that determine the intermediate tilt systems in double perovskites, we investigated, using profile analysis of X-ray diffraction data, the structures of the Ba2−xSrxBB′O6 systems with B = Ca and Y and B′ being either a d0- (W(VI)) or d10-cation (Sb(V)). Unlike the recently studied Ba2Ca1−xSrxWO6 and Ba2LnSbO6 (Ln = lanthanides and Y) systems, in which the intermediate tilt system is a−a−a− (space group R3¯), we found the tilt system a0a0c− (space group I4/m  ) occurring in both systems as the intermediate between the cubic Fm3¯m (a0a0a0) and the monoclinic I2/m (a0b−b−) structures. This observation does not support the idea that local B′–O π-bonding is the force driving the tilting, but points out that the large A-cation is more important in determining which structure is found at a given composition.