Temperature-dependent anisotropy in the bond lengths of UO 2 as a result of phonon-induced atomic correlations.

Previous experiments on cubic UO 2 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 for50<T<1023 K and produced via Fourier transform a pair-distribution functionPDF(r). ThePDF(r)shows quite clearly thatr(U-O), defined by the maximum of the U-O peak in thePDF(r), does in fact decrease with increasing temperature, whereasr(U-U)follows the lattice expansion as expected. We also observe that ther(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 theΔ5(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.