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In this work, two new perovskites of composition BaFe 0.875 Re 0.125 O 3− δ   and BaFe 0.75 Ta 0.25 O 3− 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 BaFeO 3  hexagonal perovskite (space group  P6    3   / mmc ), typified as the 6H polytype, stabilized when the perovskite tolerance factor slightly overpasses the unity. Whereas BaFe 0.875 Re 0.125 O 3− 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, BaFe 0.75 Ta 0.25 O 3− 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.

zeitschrift für kristallographie - crystalline materials

BaFe 0.875 Re 0.125 O 3− δ   and BaFe 0.75 Ta 0.25 O 3− δ   as potential cathodes for solid-oxide fuel-cells: a structural study from neutron diffraction data