# Temperature-dependent anisotropy in bond length of UO2 as a result of phonon-induced correlations

Lionel Desgranges, Gianguido Baldinozzi, Henry Fischer, Gerard H Lander

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Lionel Desgranges, Gianguido Baldinozzi, Henry Fischer, Gerard H Lander. Temperature-dependent anisotropy in bond length of UO2 as a result of phonon-induced correlations. 52nd Journées des Actinides (JdA) and 14th School on the Physics and Chemistry of the Actinides (SPCA), The Max Planck Institute for Chemical Physics of Solids, Dresden, Apr 2023, Dresden, Germany. [arXiv:0305.6840](https://arxiv.org/abs/0305.6840)

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Submitted on 25 Jan 2023

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# Temperature-dependent anisotropy in bond length of UO₂ as a result of phonon-induced correlations

L. Desgranges¹, G. Baldinozzi², H. E. Fischer³, and G. H. Lander³,⁴

1 IRESNE, CEA Cadarache, 13108 Saint-Paul-lez-Durance, France
2 CentraleSupélec, CNRS, SPMS, Université Paris-Saclay, 91190 Gif-sur-Yvette, France
3 Institut Laue Langevin, 38042 Grenoble, France
4 European Commission, Joint Research Centre, 76125 Karlsruhe, Germany

e-mail: lander@ill.fr

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 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 Δ₅(ΤΟ₁) 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 [1].

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[1] L. Desgranges et al. J. Physics: Condensed Matter **35** 10LT01 (2023)

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