Characterization of the Double Perovskite Ba2BixSc0.2Co1.8鈥?i>xO6鈭捨?/sub> (x = 0.1, 0.2)
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- 作者：Yan Li ; Jinguang Cheng ; Jie Song ; Jos茅 A. Alonso ; Maria T. Fern谩ndez-D铆az ; John B. Goodenough
- 刊名：Chemistry of Materials
- 出版年：2012
- 出版时间：November 13, 2012
- 年：2012
- 卷：24
- 期：21
- 页码：4114-4122
- 全文大小：457K
- 年卷期：v.24,no.21(November 13, 2012)
- ISSN：1520-5002
文摘
The Ba2BixSc0.2Co1.8鈥?i>xO6鈭捨?/sub> (x = 0.1 or 0.2) cubic perovskites are confirmed to be mixed-valent Co3+/Co2+ 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 Ba2BB鈥睴6鈭捨?/sub> at room temperature with B = Co, B鈥?= Bi0.2Sc0.2Co0.6, 未 鈮?1.3, and a B鈥揙 bond length a little longer than the mean B鈥测€揙 bond length. These data indicate that the room-temperature x = 0.2 phase is Ba2Co2+B鈥睴4.7 with B鈥?= (Bi5+)0.2(Sc3+)0.2(Co3+)0.6. The presence of Bi5+ was confirmed by comparing the Bi4f core-level X-ray photoelectron spectrum of the x = 0.2 sample with that of BaBiO3. BaCoO3 crystallizes in the 2H hexagonal polytype as a result of t 鈮?(A鈥揙)/[鈭?(B鈥揙)] > 1, where A鈥揙 and B鈥揙 are the equilibrium bond lengths of an ABO3 perovskite. Substitution of Bi and Sc for Co as well as the incorporation of oxygen vacancies increases the mean B鈥揙 bond length to reduce t > 1 to where cubic stacking of close-packed BaO3 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鈥揙 than B鈥测€揙 mean bond length are characteristic of oxide ions in a cubic perovskite with geometric tolerance factor t > 1 as illustrated by ferroelectric BaTiO3. Disordering of the double-perovskite oxide-ion displacements occurs over the range 350鈥?00 掳C; long-range, frustrated magnetic order occurs below a Tc 鈮?35 K. X-ray absorption spectroscopy measurements show the absence of low-spin Co3+ 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 O2, which we conjecture occurs at surface intermediate-spin Co3+ ions.