The splitting of the ³H₄ multiplet in uranium dioxide

Abstract

A set of empirical calculations for the ‘strong’ crystal field have been performed, and values of the relevant parameters for the U⁴⁺ ion in UO₂ crystals are reported. The 9-fold degeneracy of the free-ion ground state ³H₄ of an uranium ion is partially lifted, and is split into a four-level system of a singlet A₁, a doublet E, and two triplets T₁ and T₂ levels. The experimental data on the neutron spectroscopy can be fitted to theory by choosing values of the parameters as B⁴ ₀ = 893.3meV, B⁶ ₀ = 387.7meV. The low value of B⁴ ₀ thus obtained suggests that the system can adequately be treated with the 'weak' Crystal Field approach, and ground state wavefunction ³H₄ can be almost entirely expressed within a single J = 4 manifold, as is the case for lanthanide systems. The calculated Landé g value of the ground state T₂ is 0.8232, and those of the first three excited levels E, T₁, A₁ are 0.8286, 0.8389, 0.8083 respectively.