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European Commission

Short description of the work
Rafal Janicki (a), Patric Lindqvist-Reis (b)

(a) Wydzial Chemii, Uniwersytet Wroclawski, F. Joliot-Curie 14, 50-383 Wrocław, Poland
(b) Institut fuer Nukleare Entsorgung, Forschungszentrum Karlsruhe, P.O. Box 3640, 76021 Karlsruhe, Germany

Scientific objectives

The main objective of this study was to determine whether Eu(III) and Cm(III) tetracarbonate complexes, [M(CO3)4]5– and/or [M(CO3)4(H2O)]5–, exist in aqueous solution of alkali metal carbonates. In other words, what is the limiting carbonate species with the general formula [M(CO3)n]3–2n? The limiting carbonate species is defined as the maximum number of ligands (L) coordinated to the central metal (M) ion in a mononuclear complex, M(L)n.

Methods

Electronic spectroscopy is useful method for studying lanthanide and actinide complexes in solution and in the solid state. The intensity variations of f-f transitions and crystal-field splitting of 4f (5f) levels, strongly depend on the nearest surrounding of the metal ion. The influence of the second and more exterior coordination shells on the 4f electrons is rather negligible. It is for this reason, the correlation between the electronic spectrum of a complex in a crystal and that in solution may be used for to determine the stoichiometry and the structure of the lanthanide and actinide complexes in solution. Using this method, it was established that the heavy lanthanides (Dy3+-Yb3+) form primarily [Ln(CO3)4]5– species in aqueous K2CO3 solutions [1].
We have decided to use this method for studying the complexation equilibria of the curium-carbonate system in alkali metal carbonate aqueous solutions. Owing to the chemical similarities between Cm3+ and Ln3+ ions, lanthanide-carbonate complexes were also studied for comparison purposes.

Spectroscopic techniques

Time-resolved laser fluorescence spectroscopy (TRLFS), UV-Vis absorption spectroscopy, IR and Raman were used in the project to study the Cm(III) -, Eu(III) -, and Gd(III) -carbonate systems.

Preparation of doped solid samples and solutions

Crystal of [C(NH2)3]5[M(CO3)4]∙2H2O (M = Y, Lu), containing impurity ions (dopand) of either Cm3+, Eu3+, or Gd3+, and [C(NH2)3]5[Gd(CO3)4(H2O)]∙2H2O doped with Cm3+ or Eu3+, were synthesized by mixing aqueous solutions of [C(NH2)3]2CO3 (0.050 mol) and YCl3, LuCl3 or GdCl3 (0.001 mol), followed by stirring and heating until the initially formed precipitates dissolved. These clear solutions were ‘spiked’ with solutions of the impurity ions to give a concentration of Cm3+ of 2.2∙10–6 M and Eu3+ and Gd3+ of 1∙10–5 M at final pH of about 12. Large colorless crystals were formed by slow evaporation of these solutions. The corresponding deuterated compounds were also prepared and their purity checked with Raman spectroscopy.
Luminescence spectra of these compounds and aqueous Cm3+ and Eu3+ solutions at different concentrations of Na2CO3 and K2CO3, respectively, were recorded. To analyse the influence of ionic strength on the formation of different curium and europium carbonate species, spectra at different concentrations of NaCl and KCl background electrolytes were recorded.

Results and Conclusions

First we have studied luminescence properties of Eu(III) carbonate system which was used as as a reference model to elucidate the complexation of Cm(III) by carbonate anions.
The luminescence bands 5D0 → 7F0,2,3,4 of Eu(III) were normalized with respect to the magnetic dipole transition 5D0 → 7F1. It was then possible to compare the intensities of luminescence spectra of the studied Eu(III) systems in both phases. Such normalization procedure was not feasible for the Cm(III) system because in its luminescence spectrum there is only one 6D7/2 → 8S7/2 electric dipole transition. On the other hand the energy of the 6D7/2 → 8S7/2 band maximum is very sensitive to changes of the geometry and/or chemical composition of the nearest surrounding of Cm(III) cation. We attempted nevertheless to take advantage of variation of this band for qualitative interpretation the chemical changes in the Cm(III)-carbonate system.
Analysis of the 5D0 ↔ 7F0 transition usually affords information about equilibria of Eu(III) complexes in solution, because the number of bands observed in the emission, absorption, or excitation spectra may indicate the number of chemically distinct environments around Eu(III). The approximate symmetry of Eu(III) in [C(NH2)3]5[Eu(CO3)4(H2O)]∙2H2O and [C(NH2)3]5[Y0.988Eu0.012(CO3)4]∙2H2O crystals is C4 and S4, respectively. According to group theory, the 5D0 ↔ 7F0 transition is forbidden by the site group selection rules for S4 symmetry. Indeed, this transition is very weak in the spectrum of [C(NH2)3]5[Y0.988Eu0.012(CO3)4]∙2H2O. In aqueous europium(III) carbonate solution there is only one, relatively intense and broad (~20 cm–1) 7F0 ↔ 5D0 band, whose peak maximum is hypsochromically shifted with ~ 12 cm–1 as compared to the crystal. This may suggest that the analysis of 7F0 ↔ 5D0 transition is not straightforward in the case of europium(III) carbonate solutions, in particular for solutions containing tetracarbonate species. For this reason we have chosen the hypersensitive 5D0 → 7F2 transition for factor analysis, since both its shape and intensity are strongly affected by changes in the first coordination sphere of europium.
Factor analysis of the 5D0 → 7F2 luminescence spectra of the aqueous europium(III) carbonate solutions enabled calculation of the stability constants of the reaction (1):

[Eu(CO3)3]3- + (CO3)2- = [Eu(CO3)4] (1)

The analysis of the structural and spectroscopic properties of the studied systems led to the following conclusions:

  • The conditional stability constant of reaction (1) strongly depends on the temperature and the ionic strength of the solution.
  • The limiting europium(III) carbonate species in aqueous solution was determined to be [Eu(CO3)4]5–, with a molar fraction of ≤ 20% in 2M K2CO3.
  • The limiting curium(III) carbonate species in aqueous solution was determined to be [Cm(CO3)4]5– or [Cm(CO3)4(H2O)]5–; however, in this case it was difficult to derive a conditional stability constant.
  • In the europium(III) carbonate system, analysis of the integrated intensity of the 7F0 ↔ 5D2 transition may lead to incorrect conclusions if the spectrum of at least one species is unknown. This was overcome by identifying the spectra of the three tetracarbonate species in the Eu3+-doped crystals of [C(NH2)3]5[M(CO3)4]∙2H2O (M = Y, Lu) and [C(NH2)3]5[Gd(CO3)4(H2O)]∙2H2O.

References: [1] R. Janicki, P. Starynowicz, Anna Mondry, Eur. J. Inorg. Chem. 2011, 3601–3616.

Main visitor contact data
Name: Dr. Rafal Janicki
Organisation: University of Wroclaw, Faculty of Chemistry

JRP Identification
JRP nr: TALI-C01-06
JRP title: Structure and spectroscopy of Eu(III) and Cm(III) tetracarbonates
JRP scope: Scope 2: Actinide in the geological environment

Visited Associated Pooled Facility
Visited APF during the stay: KIT-INE - Laboratories
Name of the APF Contact Person: Dr. Habil. Thorsten Schäfer

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Description of the work done at the associated pooled facility
Start date of the stay: 10/7/2013
End date of the stay: 11/30/2013
Quantity of access: 39
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