Previous experiments on cubic UO2have suggested that the temperature dependences of the nearest-neighbour U-O and U-U distances aredifferent. We have acquired total-scattering neutron diffraction patterns out toQ = 23.5 Å-1for50<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 aboveT ≈ 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-rtail observed in the U-O distance distribution becoming increasingly anisotropic at higherT. These results have significance for a wide range of materials in which heavy and light atoms are combined in a simple atomic structure.
Keywords: UO2; dynamic correlations; neutron scattering; thermal anisotropy; total scattering.
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