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

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 5f(2)-->5f(2), 5f(2)-->5f(1)6d(1) electronic transitions of the (UCl6)(2-) defect cluster in the Cs2ZrCl6 host crystal. The 5f(2)-->5f(2) absorption spectrum of U4+ in gas phase has also been calculated. Comparison of the 5f(2)-->5f(2) 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 5f(1)6d(t(2g))(1) and 5f(1)6d(e(g))(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 5f(2) ground state, 1 A(1g)-->1 T-1u, is calculated to be at 32 500 cm(-1), whereas the highest electric dipole allowed zero-phonon emission from the first 5f(1)6d(t(2g))(1) excited state, which is found to be 1 E-u-->1 T-1g, 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+ 5f(1)6d(t(2g))(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. (C) 2003 American Institute of Physics.