Phase segregation in mixed Nb–Sb double perovskites Ba2LnNb1−xSbxO6

The phase composition of two series of mixed Nb5+–Sb5+ double perovskites formed between the pairs Ba2EuNbO6–Ba2PrSbO6 and Ba2NdSbO6–Ba2NdNbO6 have been studied using synchrotron X-ray powder diffraction methods. In both series extensive phase segregation is observed demonstrating limited solubility of Sb5+ in these Nb5+ perovskites, irrespective of the precise structures of the double perovskite. Evidence for a monoclinic I2/m phase in the series formed between tetragonal I4/m Ba2EuNbO6 and rhombohedral image Ba2EuNbO6 is presented. It is postulated that this phase segregation is a consequence of competing bonding requirements of the Nb5+ and Sb5+ cations associated with their electronic configurations.