23 research outputs found
Crystal and magnetic structure of the Bi 2 RuMnO 7 pyrochlore: A potential new cathode for solid oxide fuel cells
Design of Sr0.7R0.3CoO3-delta (R = Tb and Er) perovskites performing as cathode materials in solid oxide fuel cells
Sr0.7R0.3CoO3-δ (R = Tb and Er) tetragonal perovskites have been prepared and evaluated as mixed ionic-electronic cathodes for SOFC. Neutron powder diffraction (NPD) measurements evidenced that both compounds are oxygen hypo-stoichiometric with long-range order of oxygen vacancies that leads to a tetragonal perovskite-type superstructure (s.g. I4/mmm) stable within the whole temperature range under study. The oxygen vacancies located mainly in the equatorial oxygen positions exhibit large displacement factors. The high oxygen mobility in Sr0.7Tb0.3CoO3-δ was confirmed by 18O oxygen labeling followed by Secondary Ion Mass Spectrometry (SIMS) with values of oxygen self-diffusion of 1.29 × 10−10 cm2/s at 525°C. Polarization resistances with LSGM as electrolyte gave values as low as 0.011 Ω⋅cm2 and maximum output powers of 570 mW/cm2 at 850°C were obtained in test cells set in electrolyte-supported configuration. Electrical conductivity, thermal and chemical expansion and stability measurements confirm the potential of these materials as cathodes for SOFC
Topotactic Oxidation of Perovskites to Novel SrMo1-xMxO4−δ (M = Fe and Cr) Deficient Scheelite-Type Oxides
New polycrystalline SrMo1−xMxO4−δ (M = Fe and Cr) scheelite oxides have been prepared by topotactical oxidation, by annealing in air at 500 °C, from precursor perovskites with the stoichiometry SrMo1−xMxO3−δ (M = Fe and Cr). An excellent reversibility between the oxidized Sr(Mo,M)O4−δ scheelite and the reduced Sr(Mo,M)O3−δ perovskite phase accounts for the excellent behavior of the latter as anode material in solid-oxide fuel cells. A characterization by X-ray powder diffraction (XRD) and neutron powder diffraction (NPD) has been carried out to determine the crystal structure features. The scheelite oxides are tetragonal, space group I41/a (No. 88). The Rietveld-refinement from NPD data at room temperature shows evidence of oxygen vacancies in the structure, due to the introduction of Fe3+/Cr4+ cations in the tetrahedrally-coordinated B sublattice, where Mo is hexavalent. A thermal analysis of the reduced perovskite (SrMo1−xMxO3−δ) in oxidizing conditions confirms the oxygen stoichiometry obtained by NPD data; the stability range of the doped oxides, below 400–450 °C, is lower than that for the parent SrMoO3 oxide. The presence of a Mo4+/Mo5+ mixed valence in the reduced SrMo1−xMxO3−δ perovskite oxides confers greater instability against oxidation compared with the parent oxide. Finally, an XPS study confirms the surface oxidation states of Mo, Fe, and Cr in the oxidized samples SrMo0.9Fe0.1O4-δ and SrMo0.8Cr0.2O4-δ
New families of Mn+-doped SrCo1−xMxO3−δ perovskites performing as cathodes in solid-oxide fuel cells
Bi2CoO2F4 - A Polar, Ferrimagnetic Aurivillius Oxide-Fluoride.
peer reviewedAurivillius oxides have been a research focus due to their ferroelectric properties, but by replacing oxide ions by fluoride, divalent magnetic cations can be introduced, giving Bi2 MO2F4 (M = Fe, Co, and Ni). Our combined experimental and computational study on Bi2CoO2F4 indicates a low-temperature polar structure of P21 ab symmetry (analogous to ferroelectric Bi2WO6) and a ferrimagnetic ground state. These results highlight the potential of Aurivillius oxide-fluorides for multiferroic properties. Our research has also revealed some challenges associated with the reduced tendency for polar displacements in the more ionic fluoride-based systems
Bi2CoO2F4 – a polar, ferrimagnetic Aurivillius oxide-fluoride
Aurivillius oxides have been a research focus due to their ferroelectric properties, but by replacing oxide ions by fluoride, divalent magnetic cations can be introduced, giving Bi2MO2F4 (M = Fe, Co, and Ni). Our combined experimental and computational study on Bi2CoO2F4 indicates a low-temperature polar structure of P21ab symmetry (analogous to ferroelectric Bi2WO6) and a ferrimagnetic ground state. These results highlight the potential of Aurivillius oxide-fluorides for multiferroic properties. Our research has also revealed some challenges associated with the reduced tendency for polar displacements in the more ionic fluoride-based systems