Orbital hybridization ͑mixing of electron configurations of opposite parities͒ 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 5f 3 and 5f 2 6d configurations for the wave functions of open-shell electrons in the U 3+ ion. This method provides a fundamental understanding and quantitative analysis of the crystal-field induced 5f-6d mixing in U 3+ : LaCl 3 and U 3+ : CeCl 3 . The odd terms of the crystal-field interaction ͓B 3 3 ͑fd͒ and B 3 5 ͑fd͒ in C 3h site symmetry͔ selectively couple the states of the 5f 3 and 5f 2 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 ͑ECM͒ of crystal-field theory is used to calculate the crystal-field parameters of the U 3+ 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 U 3+ energy levels to the experimental absorption spectra. The configuration-mixed eigenfunctions of the U 3+ states are directly used to calculate the electric dipole transition intensities and simulate the absorption spectra where the 5f 3 and 5f 2 6d configurations overlap and the Judd-Ofelt theory fails because of significant configuration mixing.
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