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Understanding Local Structure versus Long-Range Structure: The Case of UO2.

A recent trend in the development of new optimized materials makes use of crystalline domains having nanometer sizes for which characterization methods at the atomic scale are mandatory. Amongst them is pair-distribution function analysis (PDF-analysis), a diffraction technique that has already shown that a short-range or "local" atomic structure of a given domain, having a lower symmetry than the average long-range structure, often exists in many compounds having valuable properties for industrial applications, such as pyrochlores, spinels, and doped ceria among others. However, the manner by which these domains are arranged to produce the average long-range structure is still an open question. Herein, the first structural model that accounts for both the local structure (inside a given domain) and the long-range structure (averaged over all domains) that is observed in the PDF of uranium dioxide is presented. The structural model describes domain walls in such a way as to preserve the uranium coordination polyhedron and to obey the needed symmetry rules. The proper description of domain walls is an important step in the understanding and the modelling of nanostructured materials.