Wydział Chemii

Mirosław Karbowiak

A. Mech

Janusz Drożdżyński

Z. Gajek

Norman M. Edelstein

2002

New Journal of Chemistry

26

1651-1657

10.1039/B202707K

Naukowa

Angielski

Artykuł

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 B^k_q crystal-field parameters. The value determined for the crystal-field strength parameter, N_v corresponds well with those determined for U³⁺ in other chloride single crystals.In recent years we have reported the results of crystal-field analyses for U³⁺ doped RbY₂Cl₇,¹ K₂UX₅ (X = Cl, Br or I),² Cs₂NaYCl₆³ and Cs₂LiYCl₆³ single crystals as well as for some polycrystalline samples.^4–6 Since, in all these chloride crystals, the U³⁺ ions readily undergo hydrolysis and oxidation in the presence of traces of water, we have searched for moisture resistant crystals. To date only fluoride hosts provide such properties. However, due to experimental difficulties, no high quality U³⁺ doped single crystals of a fluoride compound could be grown. Simoni et al. have converted U⁴⁺ ions doped in LiYF₄ single crystals into U³⁺ ions by γ-irradiation.⁷ This method, however, inevitably leads to a number of site defects. Another important disadvantage is that only a very small UF₄ concentration could be reduced to the U³⁺ state. Since Wickleder et al.⁸ synthesized, in a systematic search for new host lattices, a series of relatively air-stable Ba₂MCl₇ type of compounds (where M = Gd–Yb, Y) and determined their crystal structure, we turned our attention towards the relatively moisture-stable Ba₂YCl₇ crystal.Ba₂YCl₇ crystallizes in the monoclinic space group P2₁/c (No. 14).⁸ The Y³⁺ ions are seven-coordinated and the characteristic feature of the crystal structure is the [YCl₇] polyhedra, build in the form of a trigonal prism and capped on one rectangular face. Due to a slight distortion of the polyhedra, the Y³⁺ site symmetry is C₁. The [YCl₇] units are isolated with an extremely long Y³⁺–Y³⁺ distance of approximately 6.5 Å. The Y–Cl distances, approximately 2.7 Å, are typical for yttrium complex chlorides.In this paper the absorption and emission spectra for U³⁺ : Ba₂YCl₇ are presented and discussed. A crystal-field analysis, based on both the angular overlap model (AOM) and the conventional parametric Hamiltonian, is presented.

https://doi.org/10.1039/B202707K

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