Crystal-field splitting in UO$_2$

Uranium Dioxide (UO$_{2})$ is an important nuclear fuel material. We performed high-resolution inelastic neutron scattering using PHAROS at the Los Alamos spallation source LANSCE in order to re-investigate the crystal field splitting in UO$_{2}$, determined with the knowledge of the dipole-allowed transitions. We obtained the crystal field parameters and the 5$f$ electron eigen functions for UO$_{2}$. The fourth- and sixth-degree crystal field parameters were found to be V$_{4}$= -116.24 and V$_{6}$=25.78, in good agreement with previously published results by Amoretti et al. [1]. On the other hand, these previous studies did reveal four crystal-field excitations in the 150-180 meV range, only three of which can be explained by the crystal-field model. Our experiments on a different UO$_{2}$ sample show that the previously observed peak at about 180 meV is a spurious one, thus it is not intrinsic to UO$_{2}$. [1] G. Amoretti et al., Phys. Rev. B 15 (1989) 1856