BaFe0.875Re0.125O3−δ and BaFe0.75Ta0.25O3−δ as potential cathodes for solid-oxide fuel-cells: a structural study from neutron diffraction data

Abstract In this work, two new perovskites of composition BaFe0.875Re0.125O3−δ and BaFe0.75Ta0.25O3−δ, designed from ab-initio calculations to fulfill different requisites of cathode materials for solid-oxide fuel cells (SOFC), were prepared and studied from the structural point of view from neutron powder diffraction (NPD) data. They are both derivatives of BaFeO3 hexagonal perovskite (space group P6 3 /mmc), typified as the 6H polytype, stabilized when the perovskite tolerance factor slightly overpasses the unity. Whereas BaFe0.875Re0.125O3−δ keeps this structural type, as demonstrated in this crystallographic study from NPD data at 295 and 4 K, with unit-cell parameters a = 5.70177(7); c = 14.0334(2) Å at 295 K, the second material, BaFe0.75Ta0.25O3−δ, is cubic and can be defined in the Pm-3m space group, corresponding of the perovskite arystotype, with a = 4.05876(3) Å. A conspicuous oxygen deficiency is observed, with a refined stoichiometry of 2.86(3) per formula unit. The anisotropic displacement factors for oxygen atoms in this last material are flattened disks perpendicular to the (Fe,Ta)-O-(Fe,Ta) direction, suggesting a dynamic tilting of the octahedra that could be related to the oxygen motion via oxygen vacancies across the structure. This is a pre-requisite for functional mixed-ionic-electronic (MIEC) materials performing as cathodes in SOFC.