Z. Gajek

CRYSTAL-FIELD ANALYSIS OF U3+ IONS IN K2LAX5 (X = CL, BR OR I) SINGLE CRYSTALS

Abstract An analysis of low temperature absorption spectra of U3+ ions doped in K2LaX5 (X=Cl, Br or I) single crystals is reported. The energy levels of the U3+ ion in the single crystals were assigned and fitted to a semiempirical Hamiltonian representing the combined atomic and crystal-field interactions at the Cs symmetry site. An analysis of the nephelauxetic effect and crystal-field splittings in the series of compounds is also reported.

Crystal field effect in the uranium compounds— model calculations for CsUF6, Cs2UCl6 and UCl4

Abstract A practical crystal field model allowing one to estimate the crystal field parameters from first principles is presented and applied to the actinide compounds. The model results directly from the renormalization (and reduction) procedure of the true Schrodinger equation for an effective Hamiltonian acting on the 5f spin-orbitals only. In practice this approach becomes convergent with the ab initio model of Newman. Three ionic uranium compounds: CsUF 6 , Cs 2 UCl 6 and UCl 4 have served as examples of the application. The results obtained, particularly for the first two compounds, are in good agreement with the experimental data. The contributions of different mechanisms responsible for the crystal field effect are discussed.

Absorption spectra analysis of hydrated uranium(III) complex chlorides

Absorption spectra of powdered samples of hydrated uranium(III) complex chlorides of the formulas NH4UCl4 4H2O and CsUCl4 3H2O have been recorded at 4.2 K in the 4000‐26 000 cm ˇ1 range. The analysis of the spectra enabled the determination of crystal-field parameters and assignment of 83 and 77 crystal-field levels for the tetrahydrate and trihydrate, respectively. The energies of the levels were computed by applying a simplified angular overlap model as well as a semiempirical Hamiltonian representing the combined atomic and crystal-field interactions. Ab initio calculations have enabled the application of a simplified parameterization and the determination of the starting values of the AOM parameters. The received results have proved that the AOM approach can quite well predict both the structure of the ground multiplet and the positions of the crystal-field levels in the 17 000‐25 000 cm ˇ1 range, usually obscured by strong f‐d bands. ” 2000 Elsevier Science B.V. All rights reserved.

Optical spectroscopy and crystal-field analysis of U3+ : Ba2YCl7

High resolution absorption spectra of a U3+(0.3%) : Ba2YCl7 single crystal were recorded in the 4000–50 000 cm−1 range at 7 K. The observed crystal-field levels were assigned and fit to the parameters of the simplified angular overlap model (AOM) as well as a semi-empirical Hamiltonian representing the combined atomic and one-electron crystal-field interactions. The starting values of the AOM parameters were obtained from ab intio calculations. The analysis of the spectra allowed the assignment of 65 crystal-field levels with a relatively small rms deviation of 25 cm−1 and has shown that the AOM approach can predict quite well the Bkq crystal-field parameters. The value determined for the crystal-field strength parameter, Nv corresponds well with those determined for U3+ in other chloride single crystals.

Absorption spectrum analysis of uranium trichloride heptahydrate

Abstract A good quality absorption spectrum of a powdered sample of UCl 3 ·7H 2 O was obtained at 4.2 K in the 4000–30 000 cm −1 range. Analysis of the spectrum enabled the determination of the crystal-field parameters and assignment of 94 crystal-field levels. The energies of the levels were computed by applying a simplified angular overlap model as well as a semi-empirical Hamiltonian representing the combined atomic and crystal-field interactions.

Crystal structure and magnetic susceptibility of UOSe single crystals

Abstract Crystal structure and magnetic susceptibility behaviour of UOSe single crystals have been studied. UOSe crystallizes in the tetragonal PbFCl-type structure (space group P4 nmm ) with the lattice parameters: a = 390.38(5) pm and c = 698.05(9) pm. It orders antiferromagnetically at TN = 100 ± 2 K and exhibits a very strong anisotropy in the susceptibility vs temperature variation. The magnetic and thermodynamic properties of UOSe are successfully interpreted in the framework of a perturbative ab initio crystal field approach.

Reliability of conventional crystal field models in f-electron systems

Abstract Crystal field models commonly applied to explain the electronic properties of solid f-electron compounds are discussed from the point of view of their inherent limitations and the false conclusions they may lead to. Both phenomenological and ab initio approximate models are considered. The discussion is based on generalized perturbation model calculations of the crystal field parameters for europium, uranium, plutonium and neptunium ions in various crystals. The results reveal the inadequacy of various electrostatic approaches and the correctness of models based on renormalization terms.

Optical absorption spectra of the uranium (4+) ion in the thorium germanate matrix

Visible and infrared absorption measurements on the ion in tetragonal zircon-type matrix are reported and analysed in terms of the standard parametrization scheme. The observed 17 main peaks and a number of less intense lines have been assigned and fitted to most of the 32 allowed electric dipole transitions with the root mean square error equal to . The free-ion parameters obtained for the model Hamiltonian, , , and , as well as the corresponding crystal-field parameters, , , , and , agree fairly well with the initial theoretical estimations. The results are discussed in relation to the previous spectroscopic study on the scheelite-type matrix .

Interpretation of the optical absorption spectrum of uranium germanate

Visible and infrared absorption spectra of U4+ in UGeO4 are described and interpreted in terms of the standard parametrization scheme. One of the two models considered is consistent with available interpretations of other uranium germanates and silicates. The second one originating from theoretical estimation of the crystal-field effect permits exceptional properties of the compound under consideration, in particular, an untypical assignation of the electronic energy levels and values of the crystal-field parameters in comparison with other tetragonal oxides.