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Cooperative mechanisms of fast-ion conduction in gallium-based oxides with tetrahedral moieties

Nature Materials volume 6, pages 871–875 (2007)Cite this article

Abstract

The need for greater energy efficiency has garnered increasing support for the use of fuel-cell technology, a prime example being the solid-oxide fuel cell1,2. A crucial requirement for such devices is a good ionic (O2− or H+) conductor as the electrolyte3,4. Traditionally, fluorite- and perovskite-type oxides have been targeted3,4,5,6, although there is growing interest in alternative structure types for intermediate-temperature (400–700 ∘C) solid-oxide fuel cells. In particular, structures containing tetrahedral moieties, such as La1−xCaxMO4−x/2(M=Ta,Nb,P) (refs 7,8), La1−xBa1+xGaO4−x/2 (refs 9,10) and La9.33+xSi6O26+3x/2 (ref. 11), have been attracting considerable attention recently. However, an atomic-scale understanding of the conduction mechanisms in these systems is still lacking; such mechanistic detail is important for developing strategies for optimizing the conductivity, as well as identifying next-generation materials. In this context, we report a combined experimental and computational modelling study of the La1−xBa1+xGaO4−x/2 system, which exhibits both proton and oxide-ion conduction9,10. Here we show that oxide-ion conduction proceeds via a cooperative ‘cog-wheel’-type process involving the breaking and re-forming of Ga2O7 units, whereas the rate-limiting step for proton conduction is intra-tetrahedron proton transfer. Both mechanisms are unusual for ceramic oxide materials, and similar cooperative processes may be important in related systems containing tetrahedral moieties.

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Figure 1: Crystal structure of LaBaGaO4.
Figure 2: Local structure around an oxide-ion vacancy defect in the LaBaGaO4 system.
Figure 3: Lowest-energy pathway for oxygen vacancy migration in La1−xBa1+xGaO4−x/2.
Figure 4: Molecular dynamics simulation trajectories of oxide ions in three mutually perpendicular planes.
Figure 5: Proton configuration and migration in LaBaGaO4.

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Acknowledgements

This work was supported by the EPSRC (Project grant: GR/S55507/02). We would also like to thank ISIS, Rutherford Appleton Laboratory UK for access to neutron diffraction facilities.

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Authors and Affiliations

  1. Materials Chemistry, UniS Materials Institute, University of Surrey, Guildford GU2 7XH, UK

    Emma Kendrick & Peter R. Slater

  2. Institute of Pharmaceutical Innovation, University of Bradford, Bradford, BD7 1DP, UK

    John Kendrick

  3. ISIS Science Diffraction & Muon Division, CCLRC Rutherford Appleton Laboratory, Chilton, Didcot OX11 0QX, UK

    Kevin S. Knight

  4. Department of Mineralogy, The Natural History Museum, Cromwell Road, London SW7 5BD, UK

    Kevin S. Knight

  5. Department of Chemistry, University of Bath, Bath BA2 7AY, UK

    M. Saiful Islam

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  1. Emma Kendrick
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  5. Peter R. Slater
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Correspondence to M. Saiful Islam or Peter R. Slater.

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Supplementary tables 1-2 and figures 1-6 (PDF 1489 kb)

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Kendrick, E., Kendrick, J., Knight, K. et al. Cooperative mechanisms of fast-ion conduction in gallium-based oxides with tetrahedral moieties. Nature Mater 6, 871–875 (2007). https://doi.org/10.1038/nmat2039

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