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

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 & Aring;(-1) for 50 < T < 1023 K 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 asymptotic to 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.