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 Å−1 for 50 < T < 1023 K and produced via Fourier transform a pair-distribution function P D F ( r ) . The P D F ( r ) shows quite clearly that r ( U–O ) , defined by the maximum of the U–O peak in the P D F ( 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 ( T O 1 ) 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.