title: Electronic structure of U3+ in Cs3Lu2Cl9 and Cs3Y2I9 single crystals.
abstract: Low-temperature emission and polarized absorption spectra have been recorded for U(3+) ions diluted in Cs(3)Lu(2)Cl(9) and Cs(3)Y(2)I(9) host crystals. The experimental crystal-field levels were fitted to 13 parameters of a semiempirical Hamiltonian representing the combined atomic, one-electron crystal field (CF) as well as two-particle correlation crystal-field (CCF) operators. The red shift of the first f-d transitions from approximately 14,800 cm(-1) in the spectrum of U(3+):Cs(3)Lu(2)Cl(9) to as low as 11,790 cm(-1) in that of U(3+):Cs(3)Y(2)I(9) has been attributed to an increase in the covalence of the U(3+)-X(-) bonds. Comparison of the differences in the Coulomb repulsion strength between U(3+) and Er(3+) ions in Cs(3)Lu(2)Cl(9) and Cs(3)Y(2)I(9) crystals suggests that the 5f electrons of U(3+) ions are more 3d-like than 4f. The CF splitting of the (2)H(9/2) and (4)F(5/2) multiplets is unexpectedly larger for U(3+):Cs(3)Y(2)I(9) than for U(3+):Cs(3)Lu(2)Cl(9), which may be viewed as a result of the proximity of f-d states. For a correct description of the energy level structure of the (2)H(9/)(2) and (4)F(5/2) multiplets, the inclusion of CCF terms in the parametric Hamiltonian has proved to be essential. The larger f-f transition intensities for U(3+):Cs(3)Y(2)I(9) were also considered to be a consequence of the red shift of the first f-d states. The inadequacy in determination of the minor atomic parameters (other than parameters for Coulomb and spin-orbit interactions) and the insufficient inclusion of the influence of excited configuration in the applied CF Hamiltonian are assumed to be the main deficiencies preventing a better agreement between the experimental and calculated energies of CF levels.
authors.affiliation: Faculty of Chemistry, University of Wrocław, ul. F. Joliot-Curie 14, 50-383 Wrocław, Poland. karb@wchuwr.chem.uni.wroc.pl
authors.forename: Mirosław
authors.initials: M
authors.lastname: Karbowiak
authors.forename: Agnieszka
authors.initials: A
authors.lastname: Mech
authors.forename: Janusz
authors.initials: J
authors.lastname: Drozdzyński
authors.forename: Witold
authors.initials: W
authors.lastname: Ryba-Romanowski
authors.forename: Michael F
authors.initials: MF
authors.lastname: Reid