5f-6d orbital hybridization of trivalent uranium in crystals of hexagonal symmetry: Effects on electronic energy levels and transition intensities

Orbital hybridization (mixing of electron configurations of opposite parities) is analyzed in the framework of crystal-field theory with a complete diagonalization of the crystal-field Hamiltonian, including both even and odd terms of crystal-field potential, and with all basis sets of the 5 f3 and 5 f2 6d configurations for the wave functions of open-shell electrons in the U3+ ion. This method provides a fundamental understanding and quantitative analysis of the crystal-field induced 5f-6d mixing in U3+: LaCl3 and U3+: CeCl3. The odd terms of the crystal-field interaction [B33 (fd) and B35 (fd) in C3h site symmetry] selectively couple the states of the 5 f3 and 5 f2 6d configurations, inducing a shift of the energy levels and allow electric dipole transitions between the configuration-mixed states. The mixture of the 5f and 6d configurations is evaluated by introducing an index of configuration mixing. The exchange charge model (ECM) of crystal-field theory is used to calculate the crystal-field parameters of the U3+ 5f and 6d electrons in terms of point-charge electrostatic interaction and orbital overlapping and covalent effect. The initial ECM estimations of the crystal-field parameters were optimized along with free-ion parameters of the Hamiltonian in nonlinear least-squares fitting of the calculated U3+ energy levels to the experimental absorption spectra. The configuration-mixed eigenfunctions of the U3+ states are directly used to calculate the electric dipole transition intensities and simulate the absorption spectra where the 5 f3 and 5 f2 6d configurations overlap and the Judd-Ofelt theory fails because of significant configuration mixing.