RESUMO
A vacancy-ordered perovskite-type compound Ba3Fe3O8 (BaFeO2.667) was prepared by oxidizing BaFeO2.5 (P21/c) with the latter compound obtained by a spray pyrolysis technique. The structure of Ba3Fe3O8 was found to be isotypic to Ba3Fe3O7F (P21/m) and can be written as Ba3Fe3+2Fe4+1O8. Mössbauer spectroscopy and ab initio calculations were used to confirm mixed iron oxidation states, showing allocation of the tetravalent iron species on the tetrahedral site, and octahedral as well as square pyramidal coordination for the trivalent species within a G-type antiferromagnetic ordering. The uptake and release of oxygen were investigated over a broad temperature range from room temperature to 1100 °C under pure oxygen and ambient atmosphere via a combination of DTA/TG and variable temperature diffraction measurements. The compound exhibited a strong lattice enthalpy driven reduction to monoclinic and cubic BaFeO2.5 at elevated temperatures.
RESUMO
In this study, symmetrical films of BaFeO2.67, BaFeO2.33F0.33 and BaFeO2F were synthesized and the oxygen uptake and conduction was investigated by high temperature impedance spectroscopy under an oxygen atmosphere. The data were analyzed on the basis of an impedance model designed for highly porous mixed ionic electronic conducting (MIEC) electrodes. Variable temperature X-ray diffraction experiments were utilized to estimate the stability window of the oxyfluoride compounds, which yielded a degradation temperature for BaFeO2.33F0.33 of 590 °C and a decomposition temperature for BaFeO2F of 710 °C. The impedance study revealed a significant change of the catalytic behavior in dependency of the fluorine content. BaFeO2.67 revealed a bulk-diffusion limited process, while BaFeO2.33F0.33 appeared to exhibit a fast bulk diffusion and a utilization region δ larger than the electrode thickness L (8 µm). In contrast, BaFeO2F showed very area specific resistances due to the lack of oxygen vacancies. The activation energy for the uptake and conduction process of oxygen was found to be 0.07/0.29 eV (temperature range-dependent), 0.33 eV and 0.67 eV for BaFeO2.67, BaFeO2.33F0.33 and BaFeO2F, respectively.
RESUMO
In this article, we report on the synthesis and characterisation of fluorinated epitaxial films of BaFeO2F via low-temperature fluorination of thin films of BaFeO2.5+d grown by pulsed laser deposition. Diffraction measurements show that fluoride incorporation only results in a contraction of the film perpendicular to the film surface, where clamping by the substrate is prohibitive for strong in-plane changes. The fluorinated films were found to be homogenous regarding the fluorine content over the whole film thickness, and can be considered as single crystal equivalents to the bulk phase BaFeO2F. Surprisingly, fluorination resulted in the change of the tetragonal distortion to a nearly cubic symmetry, which results in a lowering of anisotropic orientation of the magnetic moments of the antiferromagnetically ordered compound, confirmed by Mössbauer spectroscopy and magnetic studies.
RESUMO
Perovskite oxides exhibiting mixed protonic and electronic conductivities have interesting applications in protonic ceramic fuel cells. In this work, we report on a hydrated phase of BaCoO1.80(OH)0.86 synthesized using nebulized spray pyrolysis. Structural analysis based on X-ray and neutron powder diffraction data showed that the compound is isotypic to BaFeO2.33(OH)0.33. The water loss behaviour was studied using simultaneous thermal analysis and high temperature X-ray diffraction, indicating that protons (respectively water) can be stabilized within the compound up to temperatures significantly above 673 K, confirmed by ex situ Fourier transform infrared spectroscopy studies. Impedance spectroscopy was used to determine the conductivity characteristics of BaCoO1.80(OH)0.86, finding and a total electrical conductivity in the order of 10-4 S cm-1 at ambient temperature with an activation energy of 0.28 eV.