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

In this work, two new perovskites of composition BaFe0.875Re0.125O3-delta and BaFe0.75Ta0.25O3-delta, 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-delta 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) angstrom at 295 K, the second material, BaFe0.75Ta0.25O3-delta, is cubic and can be defined in the Pm-3m space group, corresponding of the perovskite arystotype, with a = 4.05876(3) angstrom. 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.

Automated summary

In this study, the structural analysis of two new perovskite materials BaFe0.875Re0.125O3-delta and BaFe0.75Ta0.25O3-delta was conducted using neutron powder diffraction data. BaFe0.875Re0.125O3-delta exhibits a hexagonal structure, while BaFe0.75Ta0.25O3-delta is cubic. Both materials show oxygen deficiency, and the oxygen atoms in BaFe0.75Ta0.25O3-delta are arranged in flattened disks, which may be related to the dynamic movement of oxygen ions caused by oxygen vacancies.