Stabilization of a Complex Perovskite Superstructure under Ambient Conditions: Influence of Cation Composition and Ordering, and Evaluation as an SOFC Cathode

摘要

Ba_(1.6)Ca_(2.3)Y_(1.1)Fe5O_(13) is an Fe~(3+) oxide adopting a complex perovskite superstructure, which is an ordered intergrowth between the Ca2Fe2O5 and YBa2Fe3O8 structures featuring octahedral, square pyramidal, and tetrahedral B sites and three distinct A site environments. The distribution of A site cations was evaluated by combined neutron and X-ray powder diffraction. Consistent with the Fe~(3+) charge state, the material is an antiferromagnetic insulator with a Neel temperature of 480-485 °C and has a relatively low d.c. conductivity of 2.06 S cm~(-1) at 700 °C. The observed area specific resistance in symmetrical cell cathodes with the samarium-doped ceria electrolyte is 0.87 Ω cm~2 at 700 °C, consistent with the square pyramidal Fe~(3+) layer favoring oxide ion formation and mobility in the oxygen reduction reaction. Density functional theory calculations reveal factors favoring the observed cation ordering and its influence on the electronic structure, in particular the frontier occupied and unoccupied electronic states.