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 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.