Characterization of the Double Perovskite Ba₂Bi_xSc_0.2Co_1.8–xO_6−δ (x = 0.1, 0.2)

The Ba 2 Bi x Sc 0.2 Co 1.8– x O 6−δ ( x = 0.1 or 0.2) cubic perovskites are confirmed to be mixed-valent Co 3+ /Co 2+ oxides that, as mixed oxide-ion and electronic conductors, are superior cathode materials for an intermediate-temperature solid oxide fuel cell catalyzing the oxygen-reduction reaction. Neutron powder diffraction for x = 0.2 was used to confirm also that these compounds have the double-perovskite structure Ba 2 BB′O 6−δ at room temperature with B = Co, B′ = Bi 0.2 Sc 0.2 Co 0.6, δ ≈ 1.3, and a B–O bond length a little longer than the mean B′–O bond length. These data indicate that the room-temperature x = 0.2 phase is Ba 2 Co 2+ B′O 4.7 with B′ = (Bi 5+ ) 0.2 (Sc 3+ ) 0.2 (Co 3+ ) 0.6 . The presence of Bi 5+ was confirmed by comparing the Bi4f core-level X-ray photoelectron spectrum of the x = 0.2 sample with that of BaBiO 3 . BaCoO 3 crystallizes in the 2H hexagonal polytype as a result of t ≡ (A–O)/[√2(B–O)] > 1, where A–O and B–O are the equilibrium bond lengths of an ABO 3 perovskite. Substitution of Bi and Sc for Co as well as the incorporation of oxygen vacancies increases the mean B–O bond length to reduce t > 1 to where cubic stacking of close-packed BaO 3 planes is energetically favored over hexagonal stacking that introduces large B-cation Coulombic repulsions across a shared octahedral-site face. Cooperative oxygen displacements that create larger B–O than B′–O mean bond length are characteristic of oxide ions in a cubic perovskite with geometric tolerance factor t > 1 as illustrated by ferroelectric BaTiO 3 . Disordering of the double-perovskite oxide-ion displacements occurs over the range 350–400 °C; long-range, frustrated magnetic order occurs below a T c ≈ 35 K. X-ray absorption spectroscopy measurements show the absence of low-spin Co 3+ at room temperature, and the rate-limiting step of the oxygen-reduction reaction on these oxides acting as cathodes of a solid oxide fuel cell is shown to be electron transfer from the oxide to an adsorbed O 2, which we conjecture occurs at surface intermediate-spin Co 3+ ions.