Synthesis, structures and electrical transport properties of the La2-xSrxNiTiO6-delta (0 <= x <= 0.5) perovskite series

Perovskites (ABO(3)) with mixed transition-metal cations are of particular interest for high-temperature electrochemical and electrocatalytic devices. In this regard, the effects of Sr doping on the structural and electrical properties of the double perovskite La2NiTiO6 have been studied. High-resolution neutron powder diffraction (NPD) indicates that the symmetry of the La2-xSrxNiTiO6-delta series changes from monoclinic (space group, P2(1/n)) for low Sr contents, associated with B-site cation ordering, to orthorhombic (Pnma) for x > 0.1 with increasing B-site disorder. The substitution of Sr for La is primarily charge compensated by the creation of oxygen vacancies, as indicated by NPD and analysis of the Ni oxidation state by chemical titration thermal analysis and magnetic measurements. The system exhibits p-type electronic conductivity for oxygen partial pressures (pO(2)) > 10(-18) atm, increasing with increasing Sr content (sigma = 1.18 S cm(-1) at 900 degrees C for x = 0.5 in air). A p-n transition is observed in the range 10(-18) <= pO(2) <= 10(-19) atm. Sr-doped materials are less conductive in the n-type regime than the Sr-free parent phase; however, n-type conductivity increases with greater x in the range 0.1-0.5, most probably due to increasing reducibility associated with a higher degree of B-site disorder and defect association.