5f-electron states in uranium dioxide investigated using high-resolution neutron spectroscopy

High-resolution, high-energy-transfer, inelastic neutron scattering has been used to explore the crystal-field (CF) excitations in ${\mathrm{UO}}_2$. As all the dipole-allowed transitions within the free-ion ground manifold have been identified, the observations provide a complete determination of the crystal-field potential and 5f-electron eigenstates. The fourth- and sixth-degree CF parameters are $V_4$=-123 meV and $V_6$=26.5 meV. In spite of the strength of the CF, the ground state is accurately given by the intermediate-coupling approximation with little modification by J-mixing effects. In the antiferromagnetic phase below $T_N$=30.8 K, a splitting of the cubic CF levels, due to the combined effects of the molecular field and the distortion of the oxygen-ligand cage surrounding the ${\mathrm{U}}^{4+}$ ions, has been observed. Detailed CF calculations are presented both for the case of a double-k magnetic structure with a monoclinic distortion of the oxygen sublattice, and for a combined triple-k distortion and magnetic order. The observed splittings are shown to be more consistent with the triple-k model.