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Journal Articles

Dinuclear complexes of tetravalent cerium in an aqueous perchloric acid solution

Ikeda, Atsushi*; Tsushima, Satoru*; Hennig, C.*; Yaita, Tsuyoshi; Bernhard, G.*

Dalton Transactions, 41(24), p.7190 - 7192, 2012/06

 Times Cited Count:43 Percentile:88.57(Chemistry, Inorganic & Nuclear)

Journal Articles

Comparative investigation of the solution species [U(CO$$_{3}$$)$$_{5}$$]$$^{6-}$$ and the crystal structure of Na$$_{6}$$[U(CO$$_{3}$$)$$_{5}$$]$$cdot$$12H$$_{2}$$O

Hennig, C.*; Ikeda, Atsushi; Emmerling, F.*; Kraus, W.*; Bernhard, G.*

Dalton Transactions, 39(15), p.3744 - 3750, 2010/04

 Times Cited Count:35 Percentile:81.29(Chemistry, Inorganic & Nuclear)

The coordination of the limiting U(IV) carbonate species in aqueous solution was investigated by comparing its structure parameters with those of the complex preserved in a crystal structure. The U(IV) carbonate complex in the crystal structure was identified as a [U(CO$$_{3}$$)$$_{5}$$]$$^{6-}$$ anionic complex. This monomeric anion complex forms a network with charge compensating Na+ cations and H$$_{2}$$O ligands. U L3-edge EXAFS spectra were collected from the solid Na$$_{6}$$[U(CO$$_{3}$$)$$_{5}$$]$$cdot$$12H$$_{2}$$O and the corresponding solution. The obtained data indicate the identity of the [U(CO$$_{3}$$)$$_{5}$$]$$^{6-}$$ complex in solid and solution states. The high negative charge of the [U(CO$$_{3}$$)$$_{5}$$]$$^{6-}$$ anionic complex is compensated by Na+ cations. In the solid state the Na+ cations form a bridging network between the [U(CO$$_{3}$$)$$_{5}$$]$$^{6-}$$ anion complex, while in the liquid state they seem to be located closer at the anionic complex anion.

Journal Articles

Neptunium carbonato complexes in aqueous solution; An Electrochemical, spectroscopic, and quantum chemical study

Ikeda, Atsushi; Tsushima, Satoru*; Takao, Koichiro*; Rossberg, A.*; Funke, H.*; Scheinost, A. C.*; Bernhard, G.*; Yaita, Tsuyoshi; Hennig, C.*

Inorganic Chemistry, 48(24), p.11779 - 11787, 2009/12

 Times Cited Count:33 Percentile:79.83(Chemistry, Inorganic & Nuclear)

The electrochemical behavior and complex structure of Np carbonato complexes have been investigated in aqueous Na$$_{2}$$CO$$_{3}$$ and Na$$_{2}$$CO$$_{3}$$/NaOH solutions at different oxidation states by using cyclic voltammetry, X-ray absorption spectroscopy, and density functional theory calculations. The end-member complexes of penta- and hexavalent Np in 1.5 M Na$$_{2}$$CO$$_{3}$$ with pH = 11.7 have been determined as a transdioxo neptunyl tricarbonato complex, [NpO$$_{2}$$(CO$$_{3}$$)$$_{3}$$]$$^{n-}$$ ($$n$$ = 5 for Np$$^{rm V}$$, and 4 for Np$$^{rm VI}$$). In contrast, the electrochemical oxidation of Np$$^{rm V}$$ in a highly basic carbonate solution of 2.0M Na$$_{2}$$CO$$_{3}$$/1.0M NaOH (pH $$>$$ 13) yielded a stable heptavalent Np complex of [Np$$^{rm VII}$$ O$$_{4}$$(OH)$$_{2}$$]$$^{3-}$$, indicating that the oxidation reaction from Np$$^{rm V}$$ to Np$$^{rm VII}$$ in the carbonate solution involves a drastic structural rearrangement from the transdioxo configuration to a square-planar-tetraoxo configuration, as well as exchanging the coordinating anions from carbonate ions (CO$$_{3}$$$$^{2-}$$) to hydroxide ions (OH$$^{-}$$).

Journal Articles

Speciation and structural study of U(IV) and -(VI) in perchloric and nitric acid solutions

Ikeda, Atsushi; Hennig, C.*; Tsushima, Satoru*; Scheinost, A. C.*; Bernhard, G.*; Yaita, Tsuyoshi

Inorganic Chemistry, 48(15), p.7201 - 7210, 2009/07

 Times Cited Count:72 Percentile:94.95(Chemistry, Inorganic & Nuclear)

Speciation and complex structure of U(IV) and U(VI) are studied in aqueous acidic solutions of HClO$$_{4}$$ and HNO$$_{3}$$ by means of UV-visible-NIR and X-ray absorption spectroscopies, and density functional theory (DFT) calculations. A spherical coordinating tetravalent cation of U$$^{4+}$$ is surrounded by 9-10 water molecules in the primary coordination sphere in 1.0 M HClO$$_{4}$$, while it forms a not-well-ordered colloidal compound of UO$$_{rm 2+x}$$ (x = 0.2) mixture in a lower acidic concentration of 0.1 M HClO$$_{4}$$. U(VI) exists as a transdioxo uranyl cation, UO$$_{2}$$$$^{2+}$$, and forms a 5-fold pure hydrate complex of [UO$$_{2}$$(H$$_{2}$$O)$$_{5}$$]$$^{2+}$$ in 1.0 M HClO$$_{4}$$. The water molecules in the pure hydrate complexes of U(IV) and U(VI) are successively replaced by planar bidentate coordinate nitrate ions (NO$$_{3}$$$$^{-}$$) as a function of increasing HNO$$_{3}$$ concentration. The presence of unidentate coordinate nitrate complexes or tetranitrato U(VI) complexes is less probable in the present HNO$$_{3}$$ system.

Journal Articles

Fluorescence properties of a uranyl(V)-carbonate species [U(V)O$$_{2}$$(CO$$_{3}$$)$$_{3}$$]$$^{5-}$$ at low temperature

Grossmann, K.*; Arnold, T.*; Ikeda, Atsushi; Steudner, R.*; Geipel, G.*; Bernhard, G.*

Spectrochimica Acta, Part A, 72(3), p.449 - 453, 2009/03

Fluorescence properties of a uranyl(V) tricarbonate complex were determined at T = 153 K by time-resolved laser-induced fluorescence spectroscopy (TRLFS) for the first time. The fluorescence emission bands of the uranyl(V) tricarbonate complex were detected between 380 and 440 nm with the maxima at 404.7 nm (excitation with 255 nm) and 413.3 nm (excitation with 408 nm). Besides, the fluorescence lifetime was determined to be 120 micro-sec at 153 K.

Journal Articles

Structural determination of neptunium redox species in aqueous solutions

Ikeda, Atsushi; Hennig, C.*; Rossberg, A.*; Funke, H.*; Scheinost, A. C.*; Bernhard, G.*; Yaita, Tsuyoshi

ESRF Highlights 2008, p.99 - 100, 2009/02

Structural arrangement of Np species has been investigated in various aqueous solutions by synchrotron-based X-ray absorption fine structure (XAFS) spectroscopy. The obtained results revealed that Np(IV) dominantly forms a spherically coordinated decahydrate complex, [Np(H$$_{2}$$O)$$_{20}$$]$$^{4+}$$, while Np(V) and -(VI) form a pentahydrate neptunyl complex, [NpO$$_{2}$$(H$$_{2}$$O)$$_{5}$$]$$^{n+}$$.

Journal Articles

Electrochemical and complexation behavior of neptunium in aqueous perchlorate and nitrate solutions

Ikeda, Atsushi; Hennig, C.*; Rossberg, A.*; Funke, H.*; Scheinost, A. C.*; Bernhard, G.*; Yaita, Tsuyoshi

Inorganic Chemistry, 47(18), p.8294 - 8305, 2008/09

 Times Cited Count:81 Percentile:68.53(Chemistry, Inorganic & Nuclear)

Electrochemical and complexation properties of neptunium (Np) are investigated in aqueous perchlorate and nitrate solutions by means of cyclic voltammetry, bulk electrolysis, UV-visible absorption and Np L$$_{rm III}$$-edge X-ray absorption spectroscopies. The redox reactions of Np(III)/Np(IV) and Np(V)/Np(VI) couples are reversible or quasi-reversible, while the electrochemical reaction between Np(III/IV) and Np(V/VI) is irreversible because they undergo the structural rearrangement from spherical coordinating ions (Np$$^{3+}$$ and Np$$^{4+}$$) to transdioxo neptunyl ions (NpO$$_{2}$$$$^{n+}$$, n = 1 for Np(V) and 2 for Np(VI)). A detailed analysis on extended X-ray absorption fine structure (EXAFS) spectra suggests that Np(IV) forms a decaaquo complex of [Np(H$$_{2}$$O)$$_{10}$$]$$^{4+}$$ in 1.0 M HClO$$_{4}$$, while Np(V) and -(VI) exist dominantly as pentaaquo neptunyl complexes, [NpO$$_{2}$$(H$$_{2}$$O)$$_{5}$$]$$^{n+}$$ (n = 1 for Np(V) and 2 for Np(VI)).

Journal Articles

Structural determination of individual chemical species in a mixed system by iterative transformation factor analysis-based X-ray absorption spectroscopy combined with UV-visible absorption and quantum chemical calculation

Ikeda, Atsushi; Hennig, C.*; Rossberg, A.*; Tsushima, Satoru*; Scheinost, A. C.*; Bernhard, G.*

Analytical Chemistry, 80(4), p.1102 - 1110, 2008/01

 Times Cited Count:25 Percentile:60.29(Chemistry, Analytical)

A multitechnique approach using extended X-ray absorption fine structure spectroscopy based on iterative transformation factor analysis, UV-visible absorption spectroscopy, and quantum chemical calculations has been performed for identifying the complex structure of individual U(VI) nitrate species.

Oral presentation

Structural determination of neptunium species in aqueous solutions by EXAFS and quantum chemical calculations

Ikeda, Atsushi; Hennig, C.*; Tsushima, Satoru*; Rossberg, A.*; Scheinost, A. C.*; Bernhard, G.*; Yaita, Tsuyoshi

no journal, , 

Neptunium (93Np) is one of the most problematic nuclides in the nuclear fuel reprocessing process and the following radioactive waste disposal because of its chemical similarity to the fissile nuclides of uranium (U) and plutonium (Pu). Proper understanding of the behavior of Np in the reprocessing process or in the migration process on the geological disposal of radioactive wastes requires vast fundamental information about the chemical properties of Np in solutions. In the present study, Np solution samples with different oxidation states are electrochemically prepared in aqueous perchlorate, nitrate, and carbonate solutions, and the complex structure of Np species in the sample solutions are determined by EXAFS spectroscopy and DFT calculations. The obtained results reveal the structural difference between different Np oxidation states, as well as the different coordination behavior in each solution system.

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