Phase and valence transitions in Ba2LnSnxNb1-xO6-δ

The structures of compounds in the perovskite series Ba2LnSnxNb1-xO6-δ (Ln=Pr and Tb and x=0, 0.1, 0.2, ..., 1.0) have been examined using synchrotron X-ray and neutron diffraction. It was found that niobate members of both series feature full B-site cation ordering but that this order is lost with increasing x. X-ray absorption near-edge structure (XANES) and near-infrared spectroscopies indicate that the oxidation state of the lanthanide cations gradually changes from Ln3+ to Ln4+ with increased Sn4+ doping. This is believed to be the cause of the loss of B-site ordering. Least squares analysis of the XANES spectra suggests that the rate of the transformation of Ln3+ cations to the tetravalent state is such that the Pr series contains no oxygen vacancies while the Tb series may contain a very small amount of vacancies, with δ∼0.02. - Graphical abstract: The series Ba2LnSnxNb1-xO6-δ (Ln=Pr and Tb) has been investigated using powder diffraction and spectroscopic techniques. Similarly to the analogous Sb5+ containing compounds it was found that Sn4+ doping leads to the gradual oxidation of the Ln3+ cations to the Ln4+ state. Quantitative analysis of the Ln LIII-edge absorption spectra indicates that few or no oxygen vacancies are present