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Mesoscopic fast ion conduction in nanometre-scale planar heterostructures

Nature volume 408, pages 946–949 (2000)Cite this article

Abstract

Ion conduction is of prime importance for solid-state reactions in ionic systems, and for devices such as high-temperature batteries and fuel cells, chemical filters and sensors1,2. Ionic conductivity in solid electrolytes can be improved by dissolving appropriate impurities into the structure or by introducing interfaces that cause the redistribution of ions in the space-charge regions3,4,5,6,7,8,9,10,11. Heterojunctions in two-phase systems should be particularly efficient at improving ionic conduction3,4, and a qualitatively different conductivity behaviour is expected when interface spacing is comparable to or smaller than the width of the space-charge regions in comparatively large crystals12,13,14,15. Here we report the preparation, by molecular-beam epitaxy, of defined heterolayered films composed of CaF2 and BaF2 that exhibit ionic conductivity (parallel to the interfaces) increasing proportionally with interface density—for interfacial spacing greater than 50 nanometres. The results are in excellent agreement with semi-infinite space-charge calculations3, assuming a redistribution of fluoride ions at the interfaces. If the spacing is reduced further, the boundary zones overlap and the predicted mesoscopic size effect3,12 is observed. At this point, the single layers lose their individuality and an artificial ionically conducting material with anomalous transport properties is generated. Our results should lead to fundamental insight into ionic contact processes and to tailored ionic conductors of potential relevance for medium-temperature applications.

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Figure 1: Parallel ionic conductivity of the films.
Figure 2: Variation of ionic conductivity with N/L.
Figure 3: Comparison with space-charge calculations of the (parallel) conductivity results for the long-period films (green).
Figure 4: Comparison of conductivity profiles in the semi-infinite space-charge and mesoscale situations.

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Acknowledgements

We thank W. Kussmaul for experimental assistance, G. Bilger and H. Kerber for the performance of SIMS and X-ray pole-figure measurements, and W. Dietsche for advice.

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Author notes

  1. K. Eberman

    Present address: 3M Center, St Paul, Minnesota, 55144-1000, USA

  2. Correspondence and requests for materials should be addressed to J.M. or to U.S. .

Authors and Affiliations

  1. Max-Planck-Institut für Festkörperforschung, Stuttgart, 70569, Germany

    N. Sata, K. Eberman, K. Eberl & J. Maier

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Sata, N., Eberman, K., Eberl, K. et al. Mesoscopic fast ion conduction in nanometre-scale planar heterostructures. Nature 408, 946–949 (2000). https://doi.org/10.1038/35050047

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