Neutron-Diffraction Study of UO2: Observation of an Internal Distortion

We have deduced from measurements of the neutron elastic cross section that below the Néel temperature ($T_{\mathrm{N}}$) the oxygen atoms in U${\mathrm{O}}_2$ are shifted from their ideal fluorite positions. We find quantitative agreement with experiment by assuming a rearrangement (internal shear deformation with amplitude 0.014 Å) of the oxygen sublattice. The dominance of this deformation mode in the spin-lattice interactions suggests the presence of a non-collinear magnetic structure in U${\mathrm{O}}_2$.