Understanding Local Structure versus Long‐Range Structure: The Case of UO₂

Abstract

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.

The fifty years it has taken to understand the dynamics of UO2 in its ordered state

“…These physical properties are fundamental to understanding the complete behavior of UO 2 , and there have been a number of unusual properties measured in UO 2 that have been attributed, albeit indirectly, to the low-temperature interactions we have studied. Examples are the apparent aniso tropy in the thermal conductivity [43] and the possible disorder in the oxygen atoms at high temperature [44]. Both of these measurements need further confirmation before they can be addressed theoretically, but both have possible impacts on our understanding of the practical side of this material.…”

Temperature-induced atomic-structure modifications in UO$_{2}$ and UO$_{2+x}$

“…This type of local arrangement yields two distinct U-O distances, one shorter and one longer, rather than just one average U-O distance (as for F m3m) and results in the introduction of octahedral polyhedra into the structure. In order to agree with the long-range F m3m symmetry of UO 2 from diffraction, it has been proposed that P a3 arrangements form as nano-domains that are modulated over longer length scales in order to yield an averaged fluorite-type arrangement [32].…”

Temperature-dependent anisotropy in the bond lengths of UO2 as a result of phonon-induced atomic correlations

“…In the present paper we report results from total-scattering neutron diffraction experiments [7,8] using the D4 diffractometer [9] at the Institut Laue-Langevin, Grenoble, France, on a polycrystalline sample of UO 2 over a range of temperature from 50 to 1023 K. Our results strongly support the earlier studies, and show additionally that the anisotropy effect in U-O distances is present at all temperatures, albeit stronger at higher T. We then give a description of the possible phononinduced atomic correlations that could produce this effect. We suggest that such effects as reported here may exist in a large range of compounds where light and heavy atoms are combined in a simple atomic structure.…”