The complex formation of uranyl with chloride ions in non-aqueous solvents (acetone, tributylphosphate and methyl-i-butylketone) was studied by absorption, luminescence and MCD spectroscopy. The maximal coordination of the uranyl ion by chloride ions in these organic solvents was found to be four, which is in agreement with single crystal data. The MCD spectra unambiguously exclude the existence of [UO 2 Cl 3 ] À with D 3h coordination symmetry. For the intermediately formed complexes we propose a structure of the type: UO 2 Cl 2 (solv) 4 , with an orthorhombic symmetry. For these intermediate complexes, the 19 500-22 000 cm À1 region of the absorption spectrum exhibits an intensity enhancement with respect to that of the solvated uranyl ion. This intensity enhancement as well as the increase in total luminescence intensity is explained in terms of vibronic coupling of the first electronic state (P g ) with the asymmetric stretching (n a :a 2u ) and the bending (n b :e u ) vibrations of the uranyl ion and the out-of-plane bending mode of the equatorial ligands (n 10 :b 1u ). The spectra are dominated by transitions arising from the s u þ d u configuration. The spectroscopic properties of the UO 2 Cl 2 (TBP) x complexes are compared with those of the UO 2 (NO 3 ) 2 (TBP) 2 complex (TBP ¼ tributylphosphate).

It is shown that the sum of the intensities for magnetic dipole transitions between crystal-field components of two free-ion levels in lanthanide spectra is almost independent of the symmetry of the environment. A mean theoretical sum value of 18×10−7𝒟2, 94×10−7𝒟2, and 9×10−7𝒟2 for, respectively, the 5D1←7F0, 5D0←7F1, and 5D2←7F1 dipole strengths has been found. Experimental values of the dipole strength for Eu3+ in different lattices support within reasonable limits the theoretically derived sum rule. We therefore propose to use these magnetic dipole transitions in the Eu3+ spectrum as standards for further Judd–Ofelt parametrization.

The complex formation of the uranyl ion, UO 2 2+ , with chloride ions in acetonitrile has been investigated by factor analysis of UV-vis absorption and U L 3 edge EXAFS (extended X-ray absorption fine structure) spectra. As a function of increasing [Cl -]/[UO 2 2+ ] ratio, the five monomeric speciesand [UO 2 Cl 4 ] 2have been observed. The distances determined in the first coordination sphere are: U-O ax ) 1.77 Å, U-O H2O ) 2.43 Å, U-N MeCN ) 2.53 Å, and U-Cl ) 2.68 Å. A crystalline material has been obtained from the intermediate solution with the [Cl -]/[UO 2 2+ ] ratio of ∼2, where [UO 2 Cl 2 (H 2 O)(MeCN) 2 ] is the dominating species. The crystal structure analysis of this material revealed a tetrameric complex, [(UO 2 ) 4 (µ 2 -Cl) 4 (µ 3 -O) 2 (H 2 O) 2 (CH 3 CN) 4 ] • (CH 3 CN). The crystal data are: monoclinic, space group P2 1 /n, a ) 10.6388(5) Å, b ) 14.8441(5) Å, c ) 10.8521(5) Å, β ) 109.164(5)°, and Z ) 2. The U(VI) coordination of the solution species [UO 2 Cl 2 (H 2 O)(MeCN) 2 ] changes during the crystallization by replacing one MeCN molecule with a bridging µ 3 -O atom in the tetramer.

Locations and assignments of crystal-field levels in polarized absorption spectra (77 and 298 K) are reported for Eu3+ in the GdAl3(BO3)4 huntite matrix. The levels are analysed in terms of 20 free-ion and six crystal-field parameters (D3 symmetry). J mixing is taken into account. The influence of a parameter change on the calculated energy levels is discussed and it is shown that some parameters affect only particular levels. Spectral intensities of the 4f-4f transitions are given and parametrized using a set of Blambda kq intensity parameters.

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