[Show abstract][Hide abstract] ABSTRACT: Dicesium uranyl tetrachloride (Cs2UO2Cl4) has been a model compound for experimental and theoretical studies of electronic structure of U(VI) in the form of UO2(2+) (uranyl ion) for decades. We have obtained angle-resolved electronic structure information for oriented Cs2UO2Cl4 crystal, specifically relative energies of 5f and 6d valence orbitals probed with extraordinary energy resolution by polarization dependent high energy resolution X-ray absorption near edge structure (PD-HR-XANES) and compare these with predictions from quantum chemical Amsterdam density functional theory (ADF) and ab initio real space multiple-scattering Green's function based FEFF codes. The obtained results have fundamental value but also demonstrate an experimental approach, which offers great potential to benchmark and drive improvement in theoretical calculations of electronic structures of actinide elements.
[Show abstract][Hide abstract] ABSTRACT: A portable ultrahigh vacuum system optimized for in-situ variable temperature X-ray scattering and spectroscopy experiments at synchrotron radiation beamlines was constructed and brought into operation at the Synchrotron Radiation Facility ANKA of the Karlsruhe Institute of Technology, Germany. Here we describe the main features of the new instrument and demonstrate its capabilities. The surface morphology, structure and stoichiometry of EuSi2 nanoislands are determined by in-situ grazing incidence small angle X-ray scattering and X-ray absorption spectroscopy. A size reduction with about factor of two of the nanoislands due to silicide decomposition and Eu desorption is observed after sample annealing at 1270 K for 30 min
Journal of Synchrotron Radiation 09/2014; · 3.02 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Apart from its technological importance, plutonium (Pu) is also one of the most intriguing elements because of its non-conventional physical properties and fascinating chemistry. Those fundamental aspects are particularly interesting when dealing with the challenging study of plutonium-based nanomaterials. Here we show that ultra-small (3.2±0.9 nm) and highly crystalline plutonium oxide (PuO2) nanocrystals (NCs) can be synthesized by the thermal decomposition of plutonyl nitrate ([PuO2(NO3)2]⋅3 H2O) in a highly coordinating organic medium. This is the first example reporting on the preparation of significant quantities (several tens of milligrams) of PuO2 NCs, in a controllable and reproducible manner. The structure and magnetic properties of PuO2 NCs have been characterized by a wide variety of techniques (powder X-ray diffraction (PXRD), X-ray absorption fine structure (XAFS), X-ray absorption near edge structure (XANES), TEM, IR, Raman, UV/Vis spectroscopies, and superconducting quantum interference device (SQUID) magnetometry). The current PuO2 NCs constitute an innovative material for the study of challenging problems as diverse as the transport behavior of plutonium in the environment or size and shape effects on the physics of transuranium elements.
Chemistry - A European Journal 07/2014; · 5.93 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Recently, novel container-less laser heating experimental data have been published on the melting behaviour of pure PuO2 and PuO2-rich compositions in the uranium dioxide–plutonium dioxide system. Such data showed that previous data obtained by more traditional furnace heating techniques were affected by extensive interaction between the sample and its containment. It is therefore paramount to check whether data so far used by nuclear engineers for the uranium-rich side of the pseudo-binary dioxide system can be confirmed or not. In the present work, new data are presented both in the UO2-rich part of the phase diagram, most interesting for the uranium–plutonium dioxide based nuclear fuel safety, and in the PuO2 side. The new results confirm earlier furnace heating data in the uranium-dioxide rich part of the phase diagram, and more recent laser-heating data in the plutonium-dioxide side of the system. As a consequence, it is also confirmed that a minimum melting point must exist in the UO2–PuO2 system, at a composition between x(PuO2) = 0.4 and x(PuO2) = 0.7 and 2900 K ⩽ T ⩽ 3000 K. Taking into account that, especially at high temperature, oxygen chemistry has an effect on the reported phase boundary uncertainties, the current results should be projected in the ternary U–Pu–O system. This aspect has been extensively studied here by X-ray diffraction and X-ray absorption spectroscopy. The current results suggest that uncertainty bands related to oxygen behaviour in the equilibria between condensed phases and gas should not significantly affect the qualitative trend of the current solid–liquid phase boundaries.
Journal of Nuclear Materials 02/2014; 448(1–3):330–339. · 2.02 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In this study, uranium dicarbide (UC2) has been prepared by arc melting and heat treated under vacuum to form uranium sequicarbide (U2C3) in the presence of a second phase UC2-z. Both samples, as cast and heat treated, have been characterised by chemical analyses, X-ray diffraction (XRD), 13C magic angle spinning nuclear magnetic resonance (MAS-NMR) and by extended X-ray absorption fine structure (EXAFS). The composition, the purity, the various environments of both U and C atoms as well as the bonds length with the coordination number have been determined. By combining a long-range order method (XRD) and short-range order spectroscopy techniques (EXAFS and NMR), a unique view on the microstructure of UC2, before and after heat treatment, and of U2C3 phase has been achieved.
Journal of Alloys and Compounds 12/2013; · 2.73 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The solid-sate structures of the two uranyl peroxides studtite, [UO2(η(2)-O2)(H2O)2]·2H2O, and metastudtite [UO2(η(2)-O2)(H2O)2] have been determined by U-L3 edge extended X-ray absorption fine structure (EXAFS) spectroscopy and show that upon removal of the interstitial water in studtite there are structural changes with a small shortening of the U-Operoxo and small lengthening of the U-Oyl bonds. High-energy resolution X-Ray absorption near edge structure (HR-XANES) spectroscopy has been used to probe the differences in the local electronic structure and, supported by ab initio FEFF9.5.1 calculations, dehydration causes a shift to higher energies of the occupied O p-DOS and U d- and f-DOS of metastudtite. The HR-XANES spectrum of schoepite, [(UO2)4O(OH)6]·6H2O, has been measured as the White Line intensity can give information on the mixing of metal and ligand atomic orbitals. There is an indication for higher degree of ionicity for the U-OH bond in schoepite compared to the U-O2 bond in studtite.
[Show abstract][Hide abstract] ABSTRACT: A series of uranium carbide samples, prepared by arc melting with a C/U ratio ranging from 0.96 to 1.04, has been studied by X-ray diffraction (XRD), (13)C nuclear magnetic resonance (NMR), and extended X-ray absorption fine structure (EXAFS). XRD determines phase uniqueness and the increase of the lattice parameter versus the carbon content. In contrast, (13)C NMR detects the different carbon environments in the lattice and in this study, clearly identifies the presence of discrete peaks for carbon in the octahedral lattice site in UC and an additional peak associated with excess carbon in hyperstoichiometric samples. Two peaks associated with different levels of carbon deficiency are detected for all hypostoichiometric compositions. More than one carbon environment is always detected by (13)C NMR. This exemplifies the difficulty in obtaining a perfect stoichiometric uranium monocarbide UC1.00. The (13)C MAS spectra of uranium carbides exhibit the effects resulting from the carbon content on both the broadening of the peaks and on the Knight shift. An abrupt spectral change occurs between hypo- and hyperstoichiometric samples. The results obtained by EXAFS highlight subtle differences between the different stoichiometries, and in the hyperstoichiometric samples, the EXAFS results are consistent with the excess carbon atoms being in the tetrahedral interstitial position.
[Show abstract][Hide abstract] ABSTRACT: Several rare earth coordination compounds and the first actinide coordination compound of the recently introduced multifunctional ligand (S)P[N(Me)NC(H)Py]3 (1, Py = pyridyl) have been synthesized and characterized. The electronic and structural properties of these complexes were probed by X-ray diffraction analysis, X-ray absorption fine structure (XAFS), and advanced nuclear magnetic resonance (NMR) spectroscopy. Pulsed field-gradient spin-echo (PGSE) diffusion measurements and 1H,19F heteronuclear Overhauser spectroscopy (HOESY) revealed that the degree of ion pairing of the trivalent rare earth complexes [Ln(1)(OTf)3] (Ln = Y (2), La (3), Sm (4), and Lu (5); [OTf]− = [O3SCF3]−) depends on their metal cation ionic radii and decreases in acetonitrile solution for the smaller lanthanides. The plutonium(III) complex 6 exhibits, however, a significantly different behavior in solution and has a much stronger tendency to form solvent-separated ion pairs.
[Show abstract][Hide abstract] ABSTRACT: N-donor ligands such as n-Pr-BTP (2,6-bis(5,6-dipropyl-1,2,4-triazin-3-yl)pyridine)
studied here preferentially bind An(III) over Ln(III) in liquid-liquid separation of trivalent ac-
tinides from spent nuclear fuel. The chemical and physical processes responsible for this selec-
tivity are not yet well understood. We present systematic comparative near-edge X-ray absorp-
tion structure (XANES) spectroscopy investigations at the Gd L3 edge of [GdBTP3](NO3)3,
[Gd(BTP)3](OTf)3, Gd(NO3)3, Gd(OTf)3 and N K edge of [Gd(BTP)3](NO3)3, Gd(NO3)3
complexes. The pre-edge absorption resonance in Gd L3 edge high-energy resolution X-ray ab-
sorption near edge structure spectra (HR-XANES) is explained as arising from 2p3=2 ! 4f /5d
electronic transitions by calculations with the FEFF9.5 code. Experimental evidence is found
for higher electronic density on Gd in [Gd(BTP)3](NO3)3 and [Gd(BTP)3](OTf)3 compared to
Gd in Gd(NO3)3 and Gd(OTf)3, and on N in [Gd(BTP)3](NO3)3 compared to n-Pr-BTP. The
origin of the pre-edge structure in the N K edge XANES is explained by density functional
theory (DFT) with the ORCA code. Results at the N K edge suggest a change in ligand orbital
occupancies and mixing upon complexation but further work is necessary to interpret observed
Journal of Physics Conference Series 04/2013; 430:012115.
[Show abstract][Hide abstract] ABSTRACT: High-energy resolution X-ray absorption spectroscopy (HR-XAS) and
Resonant inelastic X-ray scattering (RIXS) combined with quantum
theoretical tools are gaining importance for understanding electronic
and coordination structures of actinide (An) and lanthanide (Ln)
materials. HR-XAS is successfully used to remove lifetime broadening by
registering the partial fluorescence yield emitted by the sample,
thereby yielding highly resolved X-ray absorption near edge structure
spectra (HR-XANES), which often display resonant features not observed
in conventional XANES. We demonstrate the structural characterization
capabilities of these novel techniques by comparative discussion of U
M4/L3-HR-XANES and L3-valence band RIXS
(L3-VB-RIXS) spectra of two model U(VI) minerals. We show
that the ab initio multiple scattering theory based code FEFF9.5 is an
effective tool for calculations of An and Ln L3-HR-XANES and
L3-RIXS spectra as it successfully reproduces dipole and
quadrupole transitions in the same spectrum.
Journal of Physics Conference Series 04/2013; 430(1):2117-.
[Show abstract][Hide abstract] ABSTRACT: X-ray absorption near edge structure (XANES) is recorded for nano- and
micro-particles formed in Mo doped UO2 thin films (TFs)
prepared by in situ sputter co-deposition and 1000°C tempering.
These UO2/Mo TFs are intended to serve as synthetic models
for epsilon-particles in spent nuclear fuel. We find that when Si is
used as substrate, nano-sized zero-valent Mo particles form as desired.
However, these are embedded in USi3, which forms at the high
temper temperatures. Micron-sized Mo-particles are formed when
SiO2 is used as substrate. Using focussed X-ray beams of
varying size (500μm, 25μm and 5μm), these particles are
characterized to be predominantly hexavalent Mo oxides, potentially with
tetrahedral coordinated [Mo(VI)O4]2- at the
surface and a MoO3-like phase in the bulk. These TFs are poor
synthetic models for spent fuel epsilon-particles but do offer the
opportunity to study changes in surface structures in response to
stress/charge as a function of particle size.
Journal of Physics Conference Series 04/2013; 430(1):2113-.
[Show abstract][Hide abstract] ABSTRACT: Cementation is an industrial scale conditioning method applied to fix
and solidify liquid low and intermediate level radioactive wastes
(LLW/ILW) prior to underground disposal in geological formations.To
assist prognosis of the long-term safety of cemented waste, alteration
of uranium doped cement productswas studied in chloride-rich solutions
relevant for final LLW/ILW disposal in rock salt. After long-time
exposure of the full-scale LLW/ILW simulates to concentrated NaCl and
MgCl2 brines, solid samples were retrieved for chemical and
mineralogical analysis with an emphasis on uranium speciation in the
corroded cement matrix.Bulk and recent spatially resolved micro(μ) U
L3-XAFS measurements point to the occurrence of a diuranate type U(VI)
phase forming throughout the corroded cement monoliths. U-enriched hot
spots with dimensions up to several tens of μm turn out to be
generally X-ray amorphous.
Journal of Physics Conference Series 04/2013; 430(1):2114-.
[Show abstract][Hide abstract] ABSTRACT: We describe the synthesis of cerium, thorium and uranium oxide nanoparticles embedded in a mesoporous
matrix as template in a kind of nanocasting technique. The solid matrix is used as a template to obtain and stabilize the actinide oxide nanoparticles. We apply high resolution transmission electron microscopy (HR-TEM) to show evidence of metal oxide incorporation into the matrix pores and analyze their structure. Measured interplanar distances and calculated lattice parameters for synthesized nanosized CeO2−x and ThO2
samples differ from their bulk crystalline counterparts. We obtain with our synthesis CeO2−x particles containing both Ce4+ and larger sized Ce3+. The lattice parameter for these ceria nanoparticles is found to be larger than the bulk value due to the presence of Ce3+ with its larger ionic radius. The presence of Ce3+ was established by means of high resolution X-ray emission spectroscopy (HRXES), applied to the investigation of nanoparticles for the first time. The ThO2 nanoparticles exhibit a decrease in interplanar distances, as one might generally expected for these nanoclusters. However, the lattice distance decrease for our particles is remarkable, up to 5%, indicating that contact with the surrounding silica matrix may exert a bond distance shortening effect such as through significant external pressure on the particle surface.
[Show abstract][Hide abstract] ABSTRACT: X-ray diffraction and X-ray absorption spectroscopy have shown that a material composed of two (Pu,Am)O2–x substoichiometric fluorite-type structures is obtained by sintering in moisturized Ar/H2. The Pu oxidation state is strictly +IV, whereas Am exhibits a +III/+IV mixed valence. Thus, the Am–O system and particularly the Am oxidation state play a major role in the sintering behavior of the Pu–Am–O system. The heating of this compound in air leads to the formation of a fluorite Pu0.80Am0.20O1.98 solid solution. The transition from a one-phase to a two-phase structure takes place at a temperature lower than 800 °C. The results were compared with previous studies on Am-bearing UO2 and ThO2, which have evidenced different behaviors.
European Journal of Inorganic Chemistry 01/2013; 2013(9). · 2.97 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The interaction of hexavalent U with a freshly synthesized nanoparticulate magnetite (FeIIFeIII2O4) (stochiometric ratio x(Fe(II)/FeTOT) = 0.25–0.33), a partly oxidized synthetic nanoparticulate magnetite (x = 0.11–0.27) and maghemite nanoparticles (x = 0–1) under anoxic conditions and exclusion of CO2 as function of pH, contact time and total U concentration (3 × 10−5 M and 1 × 10−7 M) has been examined. Short term kinetic batch experiments (contact time of 90 d) for four different pH values have been conducted. Moreover, classical batch pH sorption edges have been prepared for two different uranium concentrations for a contact time of 550 d. Spectroscopic techniques (XPS, XAS) were applied to probe for the presence and amount of reduced U on the magnetite surface. Batch kinetic studies revealed a fast initial U removal from aqueous solution with >90% magnetite associated U after 24 h within the pH range 5–11. Long-term contact time batch experiments (550 d) showed neither a U removal below pH < 3 nor a decrease in the magnetite associated U at pH ⩾ 9. Redox speciation by XPS verifies the presence of reduced U (both U(IV) and U(V) were resolved if the satellite structures were used in the fitting procedure) at the near surface of magnetite up to a contact time of 550 d and a clear correlation of the amount of available Fe(II) on the magnetite surface and the amount of reduced U is observed. XANES investigation supports presence of U(V)/U(VI) uranate and U(IV). Interpretation of the EXAFS for one sample is consistent with incorporation of U into an Fe oxide phase, after long reaction times. Thermodynamic calculations based on the experimentally determined redox potentials corroborate the spectroscopic findings of U oxidation states. Overall, the results reflect the importance of structurally bound Fe(II) as redox partner for uranyl reduction.
Geochimica et Cosmochimica Acta 11/2012; 96:154-173. · 4.25 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The electronical and structural properties of Th(0.80)Am(0.20)O(2-x) materials have been studied by the coupling of X-ray diffraction and X-ray absorption spectroscopy techniques. A substoichiometric fluorite Th(IV)(0.80)Am(III)(0.20)O(1.90) solid solution is found following sintering in moisturized Ar-H(2). In contrast, heating of this sample in air leads to a nondefective fluorite Th(IV)(0.80)Am(IV)(0.20)O(2.00) solid solution. The structures of these solid solution compounds were fully characterized by assessing the interatomic distances, the coordination numbers, and the structural disorder. The effect of the sintering atmosphere on these crystallographical parameters and on the cation valences has been determined and the capability of ThO(2) to accommodate tri- and tetravalent actinides in the fluorite structure assessed.
[Show abstract][Hide abstract] ABSTRACT: Since its inauguration in 2005, the INE-Beamline for actinide research at the synchrotron source ANKA (KIT North Campus) provides dedicated instrumentation for x-ray spectroscopic characterization of actinide samples and other radioactive materials. R&D work at the beamline focuses on various aspects of nuclear waste disposal within INE's mission to provide the scientific basis for assessing long-term safety of a final nuclear waste repository. The INE-Beamline is accessible for the actinide and radiochemistry community through the ANKA proposal system and the European Union Integrated Infrastructure Initiative ACTINET-I3. Experiments with activities up to 1 × 10(+6) times the European exemption limit are feasible within a safe but flexible containment concept. Measurements with monochromatic radiation are performed at photon energies varying between ~2.1 keV (P K-edge) and ~25 keV (Pd K-edge), including the lanthanide L-edges and the actinide M- and L3-edges up to Cf. The close proximity of the INE-Beamline to INE controlled area labs offers infrastructure unique in Europe for the spectroscopic and microscopic characterization of actinide samples. The modular beamline design enables sufficient flexibility to adapt sample environments and detection systems to many scientific questions. The well-established bulk techniques x-ray absorption fine structure (XAFS) spectroscopy in transmission and fluorescence mode have been augmented by advanced methods using a microfocused beam, including (confocal) XAFS/x-ray fluorescence detection and a combination of (micro-)XAFS and (micro-)x-ray diffraction. Additional instrumentation for high energy-resolution x-ray emission spectroscopy has been successfully developed and tested.
The Review of scientific instruments 04/2012; 83(4):043105. · 1.58 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A novel dust-free route for the preparation of (U, Am)O2−x targets has been demonstrated using a combined sol–gel and single- or double Am-infiltration process at the Minor Actinide Laboratory (MALAB) of the Institute for Transuranium Elements (ITU). Samples with 10 and 20 mol% of Am were prepared. For both Am concentrations, a single cubic phase material with a fluorite structure was observed by X-ray diffraction. X-ray absorption spectroscopy was carried out to characterize the chemical state of the metal atoms and their local crystallographic environment. The U(IV) and Am(III) valence states are predominant and the O/Am ratio is ∼1.6 for both Am contents. For the 20 mol% Am, EXAFS reveals an expansion of the Am–O (2.43 Å) bond length beyond the metal–oxygen bond length in both AmO2 (2.31 Å) and UO2 (2.35 Å).
[Show abstract][Hide abstract] ABSTRACT: Tomographic studies of granite from the sp€ o Underground Laboratory in Sweden are performed to understand the observed release of natural uranium in column studies upon application of groundwater flow. X-ray phase-sensitive tomog-raphy images reconstructed from in-line X-ray phase contrasted radiographs were compared with scanning fluorescence tomography reconstructions. The latter are based on sinograms of X-ray emission lines recorded with spatial resolution on a nanometer scale for a granite rock containing U after being subjected to a radionuclide tracer experiment. The results show that the U distribution follows microfissures or pores in the granite. Naturally occurring Th is revealed to be intimately associated with what appears to be feldspar, suggesting its being present as a dopant ion in the mineral structure. Neptunium tracer was not found in a sample prepared using FIB lift-out techniques, although the presence of Np was identified in the original granite piece during screening experiments prior to FIB-ing. The Np-containing part of the sample broke off during the preparation. Although this case study was limited to only three, # Springer-Verlag Berlin Heidelberg 2011 413 samples, initial interpretation of results confirm that Th is less likely than U to become mobile in groundwater upon drilling and excavation of emplacement caverns and shafts for a nuclear waste repository in granite bedrock.
Actinide Nanoparticle Research, 1st edited by S. N. Kalmykov and M. A. Denecke, 01/2011: chapter Speciation of actinides in granite subjected to tracer studies: pages 413-435; Springer., ISBN: 978-3-642-11431-1
[Show abstract][Hide abstract] ABSTRACT: A high energy resolution x-ray absorption near-edge structure (XANES) spectroscopy study on U4+ (UO2), U5+ ([UO2Py5][KI2Py2]), and U6+ [UO2(NO3)2(H2O)6] demonstrates the potential of this experimental technique for qualitative/semiqualitative and quantitative actinide speciation investigations. We observe a pre-edge feature with quadrupole nature in a U L3 edge partial fluorescence yield-XANES spectrum. This feature is a tool for characterizing the participation of 5f orbitals in U-O bonding. The feature origin is explained by performing calculations with the finite difference method near-edge structure code based on the multiple-scattering theory and the finite difference method.