Crystal Fields In Uo2 - Revisited

We performed inelastic neutron scattering (INS) in order to re-investigate the crystal-field ground state and the level splitting in UO2. Previous INS studies on UO2 by Amoretti et al. [Physical Review B 15, 1989, 1856] uncovered four excitations at low temperatures in the 150-180 meV range. Considering the dipole-allowed transitions, only three of these transitions could be explained by the published crystal-field model. Our INS results on a different UO2 sample revealed that the unaccounted peak at about 180 meV is a spurious one, and thus not intrinsic to UO2. In good agreement with Amoretti's results, we corroborated that the ground-state of UO2 is the Gamma(5) triplet, and we computed that the fourth- and six-order crystal field parameters are V-4 = -116 meV and V-6 = 26 meV, respectively. We also studied the INS response of the non-magnetic U0.4Th0.6O2. The splitting for this thorium-doped compound is similar to the one of UO2, which orders antiferromagnetically at low temperatures. Therefore, we can conclude that magnetic interactions only weakly perturb the energy level splitting, which is dominated by strong crystal fields.