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 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 Δ 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.