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

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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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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