Going Beyond Average Structure of Uranium Dioxide Using Neutron Total-Scattering Methods

Despite a simple fluorite structure, uranium dioxide exhibits original properties, including low temperature AFM order and high temperature predicted superionic conduction. These structural features can be characterized by scattering methods because they induce changes to the long-range average crystalline order. However, their characterization is trickier when they are related to short-range or dynamic phenomena that do not modify the long-range average structure. Total scattering methods are an option for the characterization of short-range and dynamic correlations because they probe the local instantaneous structure and thus can provide information not available by a Rietveld analysis, which describes only a time and space averaged organisation of the structure.<br/><br/>In this talk, we will present the results obtained on UO2 samples by neutron diffraction measured at D4 diffractometer at ILL from 10 to 1300K. We studied the magnetic and nuclear Pair Distribution Functions (PDF) derived from these diffraction data. Two main results will be detailed. Firstly, magnetic correlations at temperatures above TN ranging from 30 to 200K are studied using the so-called “magnetic PDF” that is produced via Fourier transform of the total magnetic diffraction intensity. Just above 30K, no long range magnetic order exists and therefore no magnetic Bragg peaks in the diffraction pattern, but rather broad features of magnetic diffuse scattering which nevertheless lead to sharp peaks in the magnetic PDF(r) that we successfully modelled using the SPINVERT software, allowing to conclude that spin-spin correlations exist above TN up to the fourth nearest neighbor. Above room temperature, only nuclear scattering is significant: the second result concerns the interpretation of the U-O distance anomalies. These distances are smaller than what expected from the average structure model. This feature was interpreted as inelastic contributions of a weakly dispersing transverse phonon branch to the equal-time correlation function, providing an explanation of the oxygen disorder mechanism at high temperature in UO2 (J. Phys.: Condens. Matter 35 (2023) 10LT01) . Recently, we observed a similar effect in CeO2.