Temperature dependent anisotropy in the bond lengths of UO2 as a result of phonon-induced atomic correlations Article - Janvier 2023

Lionel Desgranges, Gianguido Baldinozzi, Henry E Fischer, Gerard H Lander

Lionel Desgranges, Gianguido Baldinozzi, Henry E Fischer, Gerard H Lander, « Temperature dependent anisotropy in the bond lengths of UO2 as a result of phonon-induced atomic correlations  », Journal of Physics : Condensed Matter, janvier 2023, 10LT01. ISSN 0953-8984

Abstract

Previous experiments on cubic UO2 have suggested that the temperature dependences of the nearest-neighbour U-O and U-U distances are different. We have acquired total-scattering neutron diffraction patterns out to Q = 23.5 Å−1 for 50 < T < 1023K and produced via Fourier transform a pair-distribution function PDF(r). The PDF(r) shows quite clearly that r(U-O), defined by the maximum of the U-O peak in the PDF(r), does in fact decrease with increasing temperature, whereas r(U-U) follows the lattice expansion as expected. We also observe that the r(U-O) contraction accelerates continuously above T ≈ 400 K, consistent with earlier experiments by others. Furthermore, by analysing the eigenvectors of the phonon modes, we show that the ∆5 (TO1) phonon tends to separate the eight equivalent U-O distances into six shorter and two longer distances, where the longer pair contribute to a high-r tail observed in the U-O distance distribution becoming increasingly anisotropic at higher T. These results have significance for a wide range of materials in which heavy and light atoms are combined in a simple atomic structure.

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