Structure and spectroscopy of U4+ defects in Cs2ZrCl6: Ab initio theoretical studies on the 5f 2 and 5f 16d1 manifolds

Abstract

The ab initio model potential embedded cluster method, which combines the explicit treatment of quantum-mechanical embedding effects with electron correlation and spin–orbit coupling, has been applied to the calculation of the U–Cl equilibrium distances, totally symmetric vibrational frequencies, and 5f2→5f2, 5f2→5f16d1 electronic transitions of the (UCl6)2− defect cluster in the Cs2ZrCl6 host crystal. The 5f2→5f2 absorption spectrum of U4+ in gas phase has also been calculated. Comparison of the 5f2→5f2 spectra in gas phase and in Cs2ZrCl6 with experiments is used for establishing the accuracy of the methods and understanding the origins of the discrepancies between theory and experiments; the agreement between the calculated and experimental values are very satisfactory. The energies of the crystal levels of the 5f16d(t2g)1 and 5f16d(eg)1 manifolds are predicted to be 31 100–51 000 and 67 300–85 500 cm−1 above the ground state, respectively. The lowest electric dipole allowed zero-phonon absorption from the 5f2 ground state, 1 A1g→1 T1u, is calculated to be at 32 500 cm−1, whereas the highest electric dipole allowed zero-phonon emission from the first 5f16d(t2g)1 excited state, which is found to be 1 Eu→1 T1g, is calculated to be at 30 200 cm−1; this means that both of them should be observable before the sharp cutoff of the Cs2ZrCl6 host with a large gap of 2300 cm−1 between the zero-phonon absorption and emission lines. The combination of experimental spectroscopic data on Cs2ZrCl6:U4+, Cs2ZrCl6:UO22+, and Cs2UO2Cl4, with the calculated energy levels of the Cs2ZrCl6:U4+ 5f16d(t2g)1 manifold allows to discuss new possible mechanisms which could explain the observed green to blue upconversion emission of Cs2ZrCl6:U4+ crystals contaminated with UO22+. Altogether, the results in this paper demonstrate the potentiality of the wave function based methods of solid-state quantum chemistry for complementing experimental techniques in the study of actinide systems like U4+-doped Cs2ZrCl6 where hundreds of excited states are involved and their electronic structure is determined by strong spin–orbit and electron correlation interactions.