Correlation of Local Structure and Diffusion Pathways in the Modulated Anisotropic Oxide Ion Conductor CeNbO4.25

Abstract
CeNbO is reported to exhibit fast oxygen ion diffusion at moderate temperatures, making this the prototype of a new class of ion conductor with applications in a range of energy generation and storage devices. To date, the mechanism by which this ion transport is achieved has remained obscure, in part due to the long-range commensurately modulated structural motif. Here we show that CeNbO forms with a unit cell 12 times larger than the stoichiometric tetragonal parent phase of CeNbO as a result of the helical ordering of Ce and Ce ions along z. Interstitial oxygen ion incorporation leads to a cooperative displacement of the surrounding oxygen species, creating interlayer NbO connectivity by extending the oxygen coordination number to 7 and 8. Molecular dynamic simulations suggest that fast ion migration occurs predominantly within the xz plane. It is concluded that the oxide ion diffuses anisotropically, with the major migration mechanism being intralayer; however, when obstructed, oxygen can readily move to an adjacent layer along y via alternate lower energy barrier pathways.

Citation
Pramana SS, Baikie T, An T, Tucker MG, Wu J, et al. (2016) Correlation of Local Structure and Diffusion Pathways in the Modulated Anisotropic Oxide Ion Conductor CeNbO4.25. Journal of the American Chemical Society 138: 1273–1279. Available: http://dx.doi.org/10.1021/jacs.5b11373.

Acknowledgements
We gratefully acknowledge the support of the EPSRC for the award of a doctoral training account studentship for R.D.B. Additionally, we acknowledge the support of King Abdullah University of Science and Technology, who partially funded this work (S.S.P. and J.W.). We further thank STFC for the award of neutron powder diffraction beam time at the Rutherford Appleton Laboratory (ISIS) under award RB1120177. We also acknowledge the Institut Laue-Langevin (ILL) for the award of Easy access on the D2B beamline and Diamond Light Source Ltd., U.K., for access to the I11 powder diffraction beamline.

Publisher
American Chemical Society (ACS)

Journal
Journal of the American Chemical Society

DOI
10.1021/jacs.5b11373

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