Study of 5f electron states in Uranium Dioxide using Inelastic Neutron Spectroscopy

Uranium dioxide, a nuclear fuel material is one of the commonly studied actinide compounds and the nature of their 5f electrons, their degree of localization and their high-temperature thermodynamic properties have been studied in detail. Knowledge of the crystal field potential is an important part of these studies. High-resolution, high-energy transfer inelastic neutron scattering has been used to study the crystal field excitations in Uranium Dioxide. These measurements were made on PHAROS at the Los Alamos Neutron Science Center, Los Alamos National Laboratory. Since all the dipole allowed transitions in the free ion ground manifold are known, the experiments allow a complete determination of the crystal field potential and 5f-electron eigenstates in Uranium Dioxide. The ground state is accurately given by intermediate-coupling approximation with little changes by J-mixing effects. Below the Neel temperature of 30.8 K, a splitting of the cubic crystal field levels is observed. The splittings are shown to be consistent with the triple-k model.