Insights into the Electronic Structure of a U(IV) Amido and U(V) Imido Complex.

Reaction of the N-heterocylic carbene ligand i PrIm (L1 ) and lithium bis(trimethylsilyl)amide (TMSA) as a base with UCl4 resulted in U(IV) and U(V) complexes. Uranium's +V oxidation state in (HL1 )2 [U(V)(TMSI)Cl5 ] (TMSI=trimethylsilylimido) (2) was confirmed by HERFD-XANES measurements. Solid state characterization by SC-XRD and geometry optimisation of [U(IV)(L1 )2 (TMSA)Cl3 ] (1) indicated a silylamido ligand mediated inverse trans influence (ITI). The ITI was examined regarding different metal oxidation states and was compared to transition metal analogues by theoretical calculations.

HERFD-XANES and RIXS Study on the Electronic Structure of Trivalent Lanthanides across a Series of Isostructural Compounds.

We performed a systematic study of the complexes of trivalent lanthanide cations with the hydridotris(1-pyrazolyl)borato (Tp) ligand (LnTp3; Ln = La, Ce, Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb, and Lu) using both high-energy-resolution fluorescence-detected X-ray absorption near-edge structure (HERFD-XANES) and resonant inelastic X-ray scattering (RIXS) at the lanthanide L3 absorption edge. Here, we report the results obtained and we discuss them against calculations performed using density functional theory (DFT) and atomic multiplet theory. The spectral shape and the elemental trends observed in the experimental HERFD-XANES spectra are well reproduced by DFT calculations, while the pre-edge energy interval is better described by atomic multiplet theory. The RIXS data show a generally rather complex pattern that originates from the intra-atomic electron-electron interactions in the intermediate and final states, as demonstrated by the good agreement obtained with calculations using an atomic-only model of the absorber. Guided by theoretical predictions, we discuss the possible origins of the observed spectral features and the trends in energy splitting across the series. The insight into the electronic structure of trivalent lanthanide compounds demonstrated here and obtained with advanced X-ray spectroscopies coupled with theoretical calculations can be applied to any lanthanide-bearing compound and be of great interest for all research fields involving lanthanides.

Sulfidation and Reoxidation of U(VI)-Incorporated Goethite: Implications for U Retention during Sub-Surface Redox Cycling.

Over 60 years of nuclear activity have resulted in a global legacy of contaminated land and radioactive waste. Uranium (U) is a significant component of this legacy and is present in radioactive wastes and at many contaminated sites. U-incorporated iron (oxyhydr)oxides may provide a long-term barrier to U migration in the environment. However, reductive dissolution of iron (oxyhydr)oxides can occur on reaction with aqueous sulfide (sulfidation), a common environmental species, due to the microbial reduction of sulfate. In this work, U(VI)-goethite was initially reacted with aqueous sulfide, followed by a reoxidation reaction, to further understand the long-term fate of U species under fluctuating environmental conditions. Over the first day of sulfidation, a transient release of aqueous U was observed, likely due to intermediate uranyl(VI)-persulfide species. Despite this, overall U was retained in the solid phase, with the formation of nanocrystalline U(IV)O2 in the sulfidized system along with a persistent U(V) component. On reoxidation, U was associated with an iron (oxyhydr)oxide phase either as an adsorbed uranyl (approximately 65%) or an incorporated U (35%) species. These findings support the overarching concept of iron (oxyhydr)oxides acting as a barrier to U migration in the environment, even under fluctuating redox conditions.

ROBL-II at ESRF: a synchrotron toolbox for actinide research.

ROBL-II provides four different experimental stations to investigate actinide and other alpha- and beta-emitting radionuclides at the new EBS storage ring of ESRF within an energy range of 3 to 35 keV. The XAFS station consists of a highly automatized, high sample throughput installation in a glovebox, to measure EXAFS and conventional XANES of samples routinely at temperatures down to 10 K, and with a detection limit in the sub-p.p.m. range. The XES station with its five bent-crystal analyzer, Johann-type setup with Rowland circles of 1.0 and 0.5 m radii provides high-energy resolution fluorescence detection (HERFD) for XANES, XES, and RIXS measurements, covering both actinide L and M edges together with other elements accessible in the 3 to 20 keV energy range. The six-circle heavy duty goniometer of XRD-1 is equipped for both high-resolution powder diffraction as well as surface-sensitive CTR and RAXR techniques. Single crystal diffraction, powder diffraction with high temporal resolution, as well as X-ray tomography experiments can be performed at a Pilatus 2M detector stage (XRD-2). Elaborate radioprotection features enable a safe and easy exchange of samples between the four different stations to allow the combination of several methods for an unprecedented level of information on radioactive samples for both fundamental and applied actinide and environmental research.

The Application of HEXS and HERFD XANES for Accurate Structural Characterisation of Actinide Nanomaterials: The Case of ThO2.

The structural characterisation of actinide nanoparticles (NPs) is of primary importance and hard to achieve, especially for non-homogeneous samples with NPs less than 3 nm. By combining high-energy X-ray scattering (HEXS) and high-energy-resolution fluorescence-detected X-ray absorption near-edge structure (HERFD XANES) analysis, we have characterised for the first time both the short- and medium-range order of ThO2 NPs obtained by chemical precipitation. By using this methodology, a novel insight into the structures of NPs at different stages of their formation has been achieved. The pair distribution function revealed a high concentration of ThO2 small units similar to thorium hexamer clusters mixed with 1 nm ThO2 NPs in the initial steps of formation. Drying the precipitates at around 150 °C promoted the recrystallisation of the smallest units into more thermodynamically stable ThO2 NPs. HERFD XANES analysis at the thorium M4 edge, a direct probe for f states, showed variations that we have correlated with the breakdown of the local symmetry around the thorium atoms, which most likely concerns surface atoms. Together, HEXS and HERFD XANES are a powerful methodology for investigating actinide NPs and their formation mechanism.

The Application of HEXS and HERFD XANES for Accurate Structural Characterisation of Actinide Nanomaterials: The Case of ThO2.

Invited for the cover of this issue is Lucia Amidani and co-workers from the The European Synchrotron, Helmholtz Zentrum Dresden-Rossendorf, Lomonosov Moscow State University, Kurchatov Institute, and the Université Grenoble Alpes. The image depicts the atomic structure of the sample being viewed through "atomic googles", which represent the X-ray techniques used in this work. Read the full text of the article at 10.1002/chem.202003360.

Probing the Local Coordination of Hexavalent Uranium and the Splitting of 5f Orbitals Induced by Chemical Bonding.

We report here a detailed experimental and theoretical investigation of hexavalent uranium in various local configurations with a high-energy-resolution fluorescence-detected X-ray absorption near-edge structure at the U M4 edge. We show the pronounced sensitivity of the technique to the arrangement of atoms around the absorber and provide a detailed theoretical interpretation revealing the nature of spectral features. Calculations based on density functional theory and on crystal field multiplet theory indicate that for all local configurations analyzed, the main peak corresponds to nonbonding 5f orbitals, and the highest energy peak corresponds to antibonding 5f orbitals. Our findings agree with the accepted interpretation of uranyl spectral features and embed the latter in a broader field of view, which interprets the spectra of a large variety of U6+-containing samples on a common theoretical ground.

Synthesis, Characterization, and Crystal Structure of Dominant Uranium(V) Brannerites in the UTi2-xAlxO6 System.

The synthesis, characterization, and crystal structure of a novel (dominant) uranium(V) brannerite of composition U1.09(6)Ti1.29(3)Al0.71(3)O6 is reported, as determined from Rietveld analysis of the high-resolution neutron powder diffraction data. Examination of the UTi2-xAlxO6 system demonstrated the formation of brannerite-structured compounds with varying Al3+ and U5+ contents, from U0.93(6)Ti1.64(3)Al0.36(3)O6 to U0.89(6)Ti1.00(3)Al1.00(3)O6. Substitution of Al3+ for Ti4+, with U5+ charge compensation, resulted in near-linear changes in the b and c unit cell parameters and the overall unit cell volume, as expected from ionic radii considerations. The presence of U5+ as the dominant oxidation state in near-single-phase brannerite compositions was evidenced by complementary laboratory U L3-edge and high-energy-resolution fluorescence-detected U M4-edge X-ray absorption near-edge spectroscopy. No brannerite phase was found for compositions with Al3+/Ti4+ > 1, which would require a U6+ contribution for charge compensation. These data expand the crystal chemistry of uranium brannerites to the stabilization of dominant uranium(V) brannerites by the substitution of trivalent cations, such as Al3+, on the Ti4+ site.

Behavior of implanted Xe, Kr and Ar in nanodiamonds and thin graphene stacks: experiment and modeling.

Implantation and subsequent behaviour of heavy noble gases (Ar, Kr, and Xe) in few-layer graphene sheets and in nanodiamonds are studied both using computational methods and experimentally using X-ray absorption spectroscopy. X-ray absorption spectroscopy provides substantial support for Xe-vacancy (Xe-V) defects as main sites for Xe in nanodiamonds. It is shown that noble gases in thin graphene stacks distort the layers, forming bulges. The energy of an ion placed in between flat graphene sheets is notably lower than that in domains with high curvature. However, if the ion is trapped in the curved domain, considerable additional energy is required to displace it. This phenomenon is likely responsible for strong binding of noble gases implanted into disordered carbonaceous phase in meteorites (the Q-component).

Local Structure in U(IV) and U(V) Environments: The Case of U3O7.

A comprehensive analysis of X-ray absorption data obtained at the U L3-edge for a systematic series of single-valence (UO2, KUO3, UO3) and mixed-valence uranium compounds (U4O9, U3O7, U3O8) is reported. High-energy resolution fluorescence detection (HERFD) X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS) methods were applied to evaluate U(IV) and U(V) environments, and in particular, to investigate the U3O7 local structure. We find that the valence state distribution in mixed-valence uranium compounds cannot be confidently quantified from a principal component analysis of the U L3-edge XANES data. The spectral line broadening, even when applying the HERFD-XANES method, is sensibly higher (∼3.9 eV) than the observed chemical shifts (∼2.4 eV). Additionally, the white line shape and position are affected not only by the chemical state, but also by crystal field effects, which appear well-resolved in KUO3. The EXAFS of a phase-pure U3O7 sample was assessed based on an average representation of the expanded U60O140 structure. Interatomic U-O distances are found mainly to occur at 2.18 (2), 2.33 (1), and 3.33 (5) Å, and can be seen to correspond to the spatial arrangement of cuboctahedral oxygen clusters. The interatomic distances derived from the EXAFS investigation support a mixed U(IV)-U(V) valence character in U3O7.

Synthesis, Structural, and Electronic Properties of K4PuVIO2(CO3)3(cr): An Environmentally Relevant Plutonium Carbonate Complex.

The chemical properties of actinide materials are often predefined and described based on the data available for isostructural species. This is the case for potassium plutonyl (PuVI) carbonate, K4PuVIO2(CO3)3(cr), a complex relevant for nuclear technology and the environment, of which the crystallographic and thermodynamic properties of which are still lacking. We report here the synthesis and characterization of PuVI achieved by single-crystal X-ray diffraction analysis and high-energy-resolution fluorescence-detected X-ray absorption near-edge structure at the Pu M4-edge coupled with electronic structure calculations. The crystallographic properties of PuVI are compared with isostructural uranium (U) and neptunium (Np) compounds. Actinyl (AnVI) axial bond lengths, [O-AnVI-O]2+, are correlated between solid, K4AnVIO2(CO3)3(cr), and aqueous, [AnVIO2(CO3)3]4-(aq) species for the UVI-NpVI-PuVI series. The spectroscopic data are compared to KPuVO2CO3(cr) and PuIVO2(cr) to tackle the trend in the electronic structure of PuVI regarding the oxidation state changes and local structural modifications around the Pu atom.

Understanding the size effects on the electronic structure of ThO2 nanoparticles.

Developing characterization techniques and analysis methods adapted to the investigation of nanoparticles (NPs) is of fundamental importance considering the role of these materials in many fields of research. The study of actinide based NPs, despite their environmental relevance, is still underdeveloped compared to that of NPs based on stable and lighter elements. We present here an investigation of ThO2 NPs performed with High-Energy Resolution Fluorescence Detected (HERFD) X-ray Absorption Near-Edge Structure (XANES) and with ab initio XANES simulations. The first post-edge feature of Th L3 edge HERFD XANES disappears in small NPs and simulations considering non-relaxed structural models reproduce the trends observed in experimental data. Inspection of the simulations of Th atoms in the core and on the surface of the NP indeed demonstrates that the first post-edge feature is very sensitive to the lowering of the number of coordinating atoms and therefore to the more exposed Th atoms at the surface of the NP. The sensitivity of the L3 edge HERFD XANES to low coordinated atoms at the surface stems from the hybridization of the d-Density of States (DOS) of Th with both O and Th neighboring atoms. This may be a common feature to other oxide systems that can be exploited to investigate surface interactions.

High-resolution X-ray absorption spectroscopy as a probe of crystal-field and covalency effects in actinide compounds.

Applying the high-energy resolution fluorescence-detection (HERFD) mode of X-ray absorption spectroscopy (XAS), we were able to probe, for the first time to our knowledge, the crystalline electric field (CEF) splittings of the [Formula: see text] shell directly in the HERFD-XAS spectra of actinides. Using ThO2 as an example, data measured at the Th 3d edge were interpreted within the framework of the Anderson impurity model. Because the charge-transfer satellites were also resolved in the HERFD-XAS spectra, the analysis of these satellites revealed that ThO2 is not an ionic compound as previously believed. The Th [Formula: see text] occupancy in the ground state was estimated to be twice that of the Th [Formula: see text] states. We demonstrate that HERFD-XAS allows for characterization of the CEF interaction and degree of covalency in the ground state of actinide compounds as it is extensively done for 3d transition metal systems.

A Novel Metastable Pentavalent Plutonium Solid Phase on the Pathway from Aqueous Plutonium(VI) to PuO2 Nanoparticles.

Here we provide evidence that the formation of PuO2 nanoparticles from oxidized PuVI under alkaline conditions proceeds through the formation of an intermediate PuV solid phase, similar to NH4 PuO2 CO3 , which is stable over a period of several months. For the first time, state-of-the-art experiments at Pu M4 and at L3 absorption edges combined with theoretical calculations unambiguously allow to determine the oxidation state and the local structure of this intermediate phase.

Aliovalent Cation Substitution in UO2: Electronic and Local Structures of U1-yLayO2±x Solid Solutions.

For nuclear fuel related applications, the oxygen stoichiometry of mixed oxides U1-yMyO2±x is an essential property as it affects fuel properties and may endanger the safe operation of nuclear reactors. A careful review of the open literature indicates that this parameter is difficult to assess properly and that the nature of the defects, i.e., oxygen vacancies or UV, in aliovalent cation-doped UO2 is still subject to controversy. To confirm the formation of UV, we have investigated the room-temperature stable U1-yLayO2±x phase using several experimental methods (e.g., XRD, XANES, and NMR) confirmed by theoretical calculations. This paper presents the experimental proof of UV and its effect we identified in both electronic and local structure. We observe that UV is formed in quasi-equimolar proportion as LaIII in U1-yLayO2±x (y = 0.06, 0.11, and 0.22) solid solutions. The fluorite structure is maintained despite the cationic substitution, but the local structure is affected as variations of the interatomic distances are found. Therefore, we provide here the definitive proof that the substitution of UIV with LaIII is not accommodated by the creation of O vacancies as has often been assumed. The UO2 fluorite structure compensates the incorporation of an aliovalent cation by the formation of UV in quasi-equimolar proportions.

High-energy resolution X-ray absorption and emission spectroscopy reveals insight into unique selectivity of La-based nanoparticles for CO2.

The lanthanum-based materials, due to their layered structure and f-electron configuration, are relevant for electrochemical application. Particularly, La2O2CO3 shows a prominent chemoresistive response to CO2. However, surprisingly less is known about its atomic and electronic structure and electrochemically significant sites and therefore, its structure-functions relationships have yet to be established. Here we determine the position of the different constituents within the unit cell of monoclinic La2O2CO3 and use this information to interpret in situ high-energy resolution fluorescence-detected (HERFD) X-ray absorption near-edge structure (XANES) and valence-to-core X-ray emission spectroscopy (vtc XES). Compared with La(OH)3 or previously known hexagonal La2O2CO3 structures, La in the monoclinic unit cell has a much lower number of neighboring oxygen atoms, which is manifested in the whiteline broadening in XANES spectra. Such a superior sensitivity to subtle changes is given by HERFD method, which is essential for in situ studying of the interaction with CO2. Here, we study La2O2CO3-based sensors in real operando conditions at 250 °C in the presence of oxygen and water vapors. We identify that the distribution of unoccupied La d-states and occupied O p- and La d-states changes during CO2 chemoresistive sensing of La2O2CO3. The correlation between these spectroscopic findings with electrical resistance measurements leads to a more comprehensive understanding of the selective adsorption at La site and may enable the design of new materials for CO2 electrochemical applications.

Uranium Redox Transformations after U(VI) Coprecipitation with Magnetite Nanoparticles.

Uranium redox states and speciation in magnetite nanoparticles coprecipitated with U(VI) for uranium loadings varying from 1000 to 10 000 ppm are investigated by X-ray absorption spectroscopy (XAS). It is demonstrated that the U M4 high energy resolution X-ray absorption near edge structure (HR-XANES) method is capable to clearly characterize U(IV), U(V), and U(VI) existing simultaneously in the same sample. The contributions of the three different uranium redox states are quantified with the iterative transformation factor analysis (ITFA) method. U L3 XAS and transmission electron microscopy (TEM) reveal that initially sorbed U(VI) species recrystallize to nonstoichiometric UO2+x nanoparticles within 147 days when stored under anoxic conditions. These U(IV) species oxidize again when exposed to air. U M4 HR-XANES data demonstrate strong contribution of U(V) at day 10 and that U(V) remains stable over 142 days under ambient conditions as shown for magnetite nanoparticles containing 1000 ppm U. U L3 XAS indicates that this U(V) species is protected from oxidation likely incorporated into octahedral magnetite sites. XAS results are supported by density functional theory (DFT) calculations. Further characterization of the samples include powder X-ray diffraction (pXRD), scanning electron microscopy (SEM) and Fe 2p X-ray photoelectron spectroscopy (XPS).

A Johann-type X-ray emission spectrometer at the Rossendorf beamline.

This paper gives a detailed description, including equations, of the Johann-type X-ray emission spectrometer which has been recently installed and tested at the Rossendorf beamline (ROBL) of the European Synchrotron Radiation Facility. The spectrometer consists of a single spherically bent crystal analyzer and an avalanche photodiode detector positioned on the vertical Rowland cycle of 1 m diameter. The hard X-ray emission spectrometer (∼3.5-25 keV) operates at atmospheric pressure and covers the Bragg angles of 65°-89°. The instrument has been tested at high and intermediate incident energies, i.e. at the Zr K-edge and at the Au L3-edge, in the second experimental hutch of ROBL. The spectrometer is dedicated for studying actinides in materials and environmental samples by high-energy-resolution X-ray absorption and X-ray emission spectroscopies.