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Anisotropic oxygen diffusion at low temperature in perovskite-structure iron oxides

Nature Chemistry volume 2, pages 213–217 (2010)Cite this article

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Abstract

Oxygen-ion conduction in transition-metal oxides is exploited in, for example, electrolytes in solid-oxide fuel cells and oxygen-separation membranes, which currently work at high temperatures. Conduction at low temperature is a key to developing further utilization, and an understanding of the structures that enable conduction is also important to gain insight into oxygen-diffusion pathways. Here we report the structural changes observed when single-crystalline, epitaxial CaFeO2.5 thin films were changed into CaFeO2 by low-temperature reductions with CaH2. During the reduction process from the brownmillerite CaFeO2.5 into the infinite-layer structure of CaFeO2, some of the oxygen atoms are released from and others are rearranged within the perovskite-structure framework. We evaluated these changes and the reaction time they required, and found two oxygen diffusion pathways and the related kinetics at low temperature. The results demonstrate that oxygen diffusion in the brownmillerite is highly anisotropic, significantly higher along the lateral direction of the tetrahedral and octahedral layers.

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Figure 1: X-ray diffraction patterns of CaFeO2.5 thin films and schematic crystal structures of the films on the substrates.
Figure 2: Logarithmic X-ray diffraction intensity reciprocal lattice space maps of CaFeO2.5 thin films.
Figure 3: Lattice constants of brownmillerite CaFeO2.5 and infinite-layer structure CaFeO2 thin films.
Figure 4: Progressive change of X-ray diffraction patterns during the reduction of a CaFeO2.5 thin film grown on the SrTiO3 substrate.
Figure 5: X-ray diffraction patterns of CaFeO2 thin films and schematic crystal structure of the films on the substrates.

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Acknowledgements

We thank K. Yoshimura for his support during this work, and K. Matsumoto and A. Sakaiguchi for discussions. This work was supported in part by Grants-in-Aid for Scientific Research (19GS0207 and 19052004), by the Global Centers of Excellence Program ‘International Center for Integrated Research and Advanced Education in Materials Science’ and by a grant for the Joint Project of Chemical Synthesis Core Research Institutions from the Ministry of Education, Culture, Sports, Science and Technology of Japan. The work was also partly supported by a European Master programme, Master in Materials Science Exploiting European Large Scale Facilities.

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Authors and Affiliations

  1. Institute for Chemical Research, Kyoto University, Uji, Kyoto, 611-0011, Japan

    Satoru Inoue, Masanori Kawai, Noriya Ichikawa & Yuichi Shimakawa

  2. Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto, 615-8510, Japan

    Hiroshi Kageyama

  3. Université de Rennes 1, Sciences Chimiques de Rennes – UMR 6226, Campus de Beaulieu, Bât 10B, Rennes, F-35042, France

    Werner Paulus

Authors
  1. Satoru Inoue
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  2. Masanori Kawai
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  4. Hiroshi Kageyama
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  5. Werner Paulus
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  6. Yuichi Shimakawa
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Contributions

S.I. and Y.S. conceived and designed the study. S.I. and M.K. performed the experiments with the help of N.I. H.K. and W.P. contributed the main discussion on oxygen diffusion in iron oxides. All of the authors discussed the results. S.I. and Y.S. wrote the manuscript with comments from H.K. and W.P.

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Correspondence to Yuichi Shimakawa.

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Inoue, S., Kawai, M., Ichikawa, N. et al. Anisotropic oxygen diffusion at low temperature in perovskite-structure iron oxides. Nature Chem 2, 213–217 (2010). https://doi.org/10.1038/nchem.547

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