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Probing disorder in isometric pyrochlore and related complex oxides

Abstract

There has been an increased focus on understanding the energetics of structures with unconventional ordering (for example, correlated disorder that is heterogeneous across different length scales1). In particular, compounds with the isometric pyrochlore structure2, A2B2O7, can adopt a disordered, isometric fluorite-type structure, (A, B)4O7, under extreme conditions3,4,5,6,7. Despite the importance of the disordering process there exists only a limited understanding of the role of local ordering on the energy landscape. We have used neutron total scattering to show that disordered fluorite (induced intrinsically by composition/stoichiometry or at far-from-equilibrium conditions produced by high-energy radiation) consists of a local orthorhombic structural unit that is repeated by a pseudo-translational symmetry, such that orthorhombic and isometric arrays coexist at different length scales. We also show that inversion in isometric spinel occurs by a similar process. This insight provides a new basis for understanding order-to-disorder transformations important for applications such as plutonium immobilization4, fast ion conduction8, and thermal barrier coatings9,10.

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Figure 1: Neutron PDFs and structural models for ordered (Ho2Ti2O7) and disordered (Ho2Zr2O7) pyrochlore.
Figure 2: Neutron powder diffraction pattern (open circles) of Ho2Zr2O7 refined with the isometric disordered fluorite structure (solid red line).
Figure 3: Comparison of neutron PDFs (open circles) for intrinsically and extrinsically disordered pyrochlores.
Figure 4: Neutron PDFs and structural models for normal (MgAl2O4) and inverse (NiAl2O4) spinel.

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Acknowledgements

We thank T. Egami, A. Fuentes and B. Haberl for discussions. This work was supported as part of the Materials Science of Actinides, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Basic Energy Sciences under Award # DE-SC0001089. This research at ORNL’s Spallation Neutron Source was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, US Department of Energy.

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J.S., C.L.T., R.C.E. and M.L. conceived and designed the experiments. M.L. and C.L.T. designed and coordinated the irradiation procedure. J.S., F.Z., S.F., D.B. and H.Z. synthesized the samples. J.S., M.F., C.L.T., J.N. and M.L. collected and interpreted the neutron scattering data. J.S., C.L.T., R.C.E. and M.L. prepared the manuscript.

Corresponding author

Correspondence to Maik Lang.

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The authors declare no competing financial interests.

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Shamblin, J., Feygenson, M., Neuefeind, J. et al. Probing disorder in isometric pyrochlore and related complex oxides. Nature Mater 15, 507–511 (2016). https://doi.org/10.1038/nmat4581

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