School of Chemistry, The University of Sydney
○Paul J Saines Brendan J. Kennedy

Double perovskites are of interest due to their wide ranging physical properties, which in many cases can be tuned by suitable chemical substitutions. The electronic and ionic conductivity of perovskites that are either oxygen deficient and/or contain mixed valent cations are of particular interest. The potential for oxygen vacancies to concentrate on a certain site in these materials is of particular interest as it could lead to the development of anisotropic ionic conductivity and hence more efficient electrolytes.

Two series of potentially oxygen deficient double perovskites Ba₂LnSn_xSb_1-xO_6-δ; (Ln = Pr and Nd) have been structurally characterized using a combination of powder neutron and synchrotron X-ray diffraction. The neodymium containing compounds were found to feature a phase transition from rhombohedral to monoclinic symmetry upon partial replacement of the Sb(V) with Sn(IV). Ordering of the resulting oxygen vacancies was observed using neutron diffraction at temperatures of up to 1073 K. The oxygen vacancies in Ba₂NdSnO_5.5 were found to concentrate on the equatorial sites at ambient conditions shifting to the axial sites at higher temperatures. This rearrangement can be explained using crystal chemistry. By comparison the symmetry of the praseodymium compounds changes from rhombohedral to monoclinic and then to tetragonal with increasing Sn(IV) content. The three B-type cations (Pr, Sb and Sn) are disordered in the tetragonal structure. Thermal-gravimetric and spectroscopic analysis suggest that increasing the tin content results in oxidation of Pr³⁺ to Pr⁴⁺ rather than the formation of oxygen vacancies.