High-energy resolution X-ray spectroscopy at actinide M4,5 and ligand K edges: what we know, what we want to know, and what we can know

Progress in Nonaqueous Molecular Uranium Chemistry: Where to Next?

There is long-standing interest in nonaqueous uranium chemistry because of fundamental questions about uranium's variable chemical bonding and the similarities of this pseudo-Group 6 element to its congener d-block elements molybdenum and tungsten. To provide historical context, with reference to a conference presentation slide presented around 1988 that advanced a defining collection of top targets, and the challenge, for synthetic actinide chemistry to realize in isolable complexes under normal experimental conditions, this Viewpoint surveys progress against those targets, including (i) CO and related π-acid ligand complexes, (ii) alkylidenes, carbynes, and carbidos, (iii) imidos and terminal nitrides, (iv) homoleptic polyalkyls, -alkoxides, and -aryloxides, (v) uranium-uranium bonds, and (vi) examples of topics that can be regarded as branching out in parallel from the leading targets. Having summarized advances from the past four decades, opportunities to build on that progress, and hence possible future directions for the field, are highlighted. The wealth and diversity of uranium chemistry that is described emphasizes the importance of ligand-metal complementarity in developing exciting new chemistry that builds our knowledge and understanding of elements in a relativistic regime.

Pyridine Diphosphonate Ligand for Stabilization of Tetravalent Uranium and Neptunium in Aqueous Medium under Aerobic Conditions

Though uranium is usually present in its +6 oxidation state (as uranyl ion) in aqueous solutions, its conversion to oxidation states such as +4 or +5 is a challenging task. Electrochemical reduction and axial oxo activation are the preferred methods to get stable unusual oxidation states of uranium in an aqueous medium. In previous studies, dicarboxylic acid has been used to stabilize UO2+ in aqueous alkaline solutions. In the present work, a diphosphonate ligand was chosen due to its higher complexing ability compared to that of the carboxylate ligands. Neptunium complexation studies with 2,6-pyridinediphosphonic acid (PyPOH) indicated the formation of different species at different pH values and the complexation facilitates disproportionation of NpO2+ to Np4+ and NpO22+ at pH 2. Hexavalent actinides form insoluble complexes in aqueous media at pH = 2, as confirmed by UO22+ complexation studies. The in situ complexation-driven precipitation resulted in conversion to pure Np4+ in aqueous media as the Np4+-PyPOH complex. A strong complexing ability of the PyPOH ligand toward the Np4+ ion is also seen for the stabilization of the electrochemically generated U4+ in aqueous medium under aerobic conditions. The U4+-PyPOH complex was found to be stable for 3 months. Raman, UV-vis, fluorescence, and cyclic voltametric studies along with density functional theory (DFT) calculations were done to get structural insights into the PyPOH complexes of actinides in different oxidation states.

Elucidating uranium interactions with synthetic Na-P1 zeolite/Ca2+-substituted alginate composite granules through batch and spectroscopic studies: Emphasizing the significance of ion exchange and complexation

Uranium, a key member of the actinides series, is radioactive and may cause severe environmental hazards once discharged into the water due to high toxicity. Removal of uranium via adsorption by applying tailored, functional adsorbents is at the forefront of tackling such pollution. Here, we report the optimized functionalization of the powder coal fly-ash (CFA) derived Na-P1 synthetic zeolite to the form of granules by employing the biodegradable polymer-calcium alginate (CA) and their application to remove aqueous U. The optimized synthesis showed that granules are formed at the CA concentration equals to 0.5 % wt., and that application of 1% wt. solution renders the most effective U scavengers. The maximum U adsorption capacity (qmax) increases significantly after CA modification from 44.48 mgU/g for native, powder Na-P1 zeolite to 62.53 mg U/g and 76.70 mg U/g for 0.5 % wt. and 1 % wt. CA respectively. The U adsorption follows the Radlich-Peterson isotherm model, being the highest at acidic pH (pHeq∼4). The U adsorption kinetics reveals swift U uptake, reaching equilibrium after 2h for 1 % ZACB and 3 h for 0.5 % wt. ZACB following the pseudo-second-order (PSO) kinetic model. SEM-EDXS investigation elucidates that adsorbed U occurs onto materials as an inhomogenous, well-dispersed, and micrometer-scale aggregate. Further, XPS and μ-XRF spectroscopies complementarily confirmed the hexavalent oxidation state of adsorbed U and its altered distribution on ZACBs with varying CA concentrations. U distribution was probed "in-situ" onto materials while correlations between the major elements (Al, Si, Ca, U) contributing to U scavenging were calculated and compared. Finally, a real-life coal mine wastewater (CMW) polluted by 238U and 228,226Ra was successfully purified, satisfying WHO guidelines after treatment using ZACBs. These findings offer new insights on successful yet optimized Na-P1 zeolite modification using biodegradable polymer (Ca2+-exchanged alginate) aimed at efficient U removal, displaying a near-zero environmental impact.

Spatiotemporal Studies of Soluble Inorganic Nanostructures with X-rays and Neutrons

This Review addresses the use of X-ray and neutron scattering as well as X-ray absorption to describe how inorganic nanostructured materials assemble, evolve, and function in solution. We first provide an overview of techniques and instrumentation (both large user facilities and benchtop). We review recent studies of soluble inorganic nanostructure assembly, covering the disciplines of materials synthesis, processes in nature, nuclear materials, and the widely applicable fundamental processes of hydrophobic interactions and ion pairing. Reviewed studies cover size regimes and length scales ranging from sub-Ångström (coordination chemistry and ion pairing) to several nanometers (molecular clusters, i.e. polyoxometalates, polyoxocations, and metal-organic polyhedra), to the mesoscale (supramolecular assembly processes). Reviewed studies predominantly exploit 1) SAXS/WAXS/SANS (small- and wide-angle X-ray or neutron scattering), 2) PDF (pair-distribution function analysis of X-ray total scattering), and 3) XANES and EXAFS (X-ray absorption near-edge structure and extended X-ray absorption fine structure, respectively). While the scattering techniques provide structural information, X-ray absorption yields the oxidation state in addition to the local coordination. Our goal for this Review is to provide information and inspiration for the inorganic/materials science communities that may benefit from elucidating the role of solution speciation in natural and synthetic processes.

Effect of carbon content on electronic structure of uranium carbides

The electronic structure of UC[Formula: see text] (x = 0.9, 1.0, 1.1, 2.0) was studied by means of x-ray absorption spectroscopy (XAS) at the C K edge and measurements in the high energy resolution fluorescence detection (HERFD) mode at the U [Formula: see text] and [Formula: see text] edges. The full-relativistic density functional theory calculations taking into account the [Formula: see text] Coulomb interaction U and spin-orbit coupling (DFT+U+SOC) were also performed for UC and UC[Formula: see text]. While the U [Formula: see text] HERFD-XAS spectra of the studied samples reveal little difference, the U [Formula: see text] HERFD-XAS spectra show certain sensitivity to the varying carbon content in uranium carbides. The observed gradual changes in the U [Formula: see text] HERFD spectra suggest an increase in the C 2p-U 5f charge transfer, which is supported by the orbital population analysis in the DFT+U+SOC calculations, indicating an increase in the U 5f occupancy in UC[Formula: see text] as compared to that in UC. On the other hand, the density of states at the Fermi level were found to be significantly lower in UC[Formula: see text], thus affecting the thermodynamic properties. Both the x-ray spectroscopic data (in particular, the C K XAS measurements) and results of the DFT+U+SOC calculations indicate the importance of taking into account U and SOC for the description of the electronic structure of actinide carbides.

Laboratory-based X-ray spectrometer for actinide science

X-ray absorption and emission spectroscopies nowadays are advanced characterization methods for fundamental and applied actinide research. One of the advantages of these methods is to reveal slight changes in the structural and electronic properties of radionuclides. The experiments are generally carried out at synchrotrons. However, considerable progress has been made to construct laboratory-based X-ray spectrometers for X-ray absorption and emission spectroscopies. Laboratory spectrometers are reliable, effective and accessible alternatives to synchrotrons, especially for actinide research, which allow dispensing with high costs of the radioactive sample transport and synchrotron time. Moreover, data from laboratory spectrometers, obtained within a reasonable time, are comparable with synchrotron results. Thereby, laboratory spectrometers can complement synchrotrons or can be used for preliminary experiments to find perspective samples for synchrotron experiments with better resolution. Here, the construction and implementation of an X-ray spectrometer (LomonosovXAS) in Johann-geometry at a radiochemistry laboratory is reported. Examples are given of the application of LomonosovXAS to actinide systems relevant to the chemistry of f-elements, the physical chemistry of nuclear power engineering and the long-term disposal of spent nuclear fuel.

A multimodal X-ray spectroscopy investigation of uranium speciation in ThTi2O6 compounds with the brannerite structure

ThTi2O6 derived compounds with the brannerite structure were designed, synthesised, and characterised with the aim of stabilising incorporation of U5+ or U6+, at dilute concentration. Appropriate charge compensation was targeted by co-substitution of Gd3+, Ca2+, Al3+, or Cr3+, on the Th or Ti site. U L3 edge X-ray Absorption Near Edge Spectroscopy (XANES) and High Energy Resolution Fluorescence Detected U M4 edge XANES evidenced U5+ as the major oxidation state in all compounds, with a minor fraction of U6+ (2-13%). The balance of X-ray and Raman spectroscopy data support uranate, rather than uranyl, as the dominant U6+ speciation in the reported brannerites. It is considered that the U6+ concentration was limited by unfavourable electrostatic repulsion arising from substitution in the octahedral Th or Ti sites, which share two or three edges, respectively, with neighbouring polyhedra in the brannerite structure.

A computational study on the coordination modes and electron absorption spectra of the complexes U(iv) with N,N,N',N'-tetramethyl-diglycolamide and anions

Lipophilic N,N,N',N'-tetraalkyl-diglycolamides (TRDGAs) are promising extractants for actinides separation in spent nuclear fuel reprocessing. Usually, in the extracted complexes of actinide and lanthanide ions of various oxidation states, the metal ions are completely surrounded by 2 or 3 TRDGA molecules, and the counter anions do not directly coordinate with them. In contrast, the extracted complexes of U(iv) from different media presenting different absorption spectra indicate that the anions (Cl- and NO3-) are directly involved in the coordination with U(iv) in the first inner sphere. Based on this exceptional observation in solvent extraction, taking the coordination of U(iv) with N,N,N',N'-tetramethyl-diglycolamide (TMDGA, the smallest analogue of TRDGA) as the research object, we mimic the behaviours of counterions (Cl- and NO3-) and the water molecule during coordination of TMDGA with U(iv), especially combining with the simulation of the absorption spectra. We demonstrate that during the complexing of TMDGA to U(iv), the counterion Cl- will occupy one coordination number in the inner coordination sphere, and NO3- will occupy two by bidentate type; however, the ubiquitous water cannot squeeze in the inner coordination sphere. In addition, the coordination of Cl- and NO3- is proved to favour the extraction with the lower binding energy. Moreover, the simulation of absorption spectra is in good agreement with the observation from experiments, further verifying the aforementioned conclusion. This work in some way will provide guidance to improve the computation methods in research of actinides by mimicking the absorption spectra of actinide ions in different complexes.

Core Excitations of Uranyl in Cs2UO2Cl4 from Relativistic Embedded Damped Response Time-Dependent Density Functional Theory Calculations

X-ray spectroscopies, by their high selectivity and sensitivity to the chemical environment around the atoms probed, provide significant insights into the electronic structures of molecules and materials. Interpreting experimental results requires reliable theoretical models, accounting for environmental, relativistic, electron correlation, and orbital relaxation effects in a balanced manner. In this work, we present a protocol for the simulation of core excited spectra with damped response time-dependent density functional theory based on the Dirac-Coulomb Hamiltonian (4c-DR-TD-DFT), in which environmental effects are accounted for through the frozen density embedding (FDE) method. We showcase this approach for the uranium M4- and L3-edges and oxygen K-edge of the uranyl tetrachloride (UO2Cl42-) unit as found in a host Cs2UO2Cl4 crystal. We have found that the 4c-DR-TD-DFT simulations yield excitation spectra that very closely match the experiment for the uranium M4-edge and the oxygen K-edge, with good agreement for the broad experimental spectra for the L3-edge. By decomposing the complex polarizability in terms of its components, we have been able to correlate our results with angle-resolved spectra. We have observed that for all edges, but in particular the uranium M4-edge, an embedded model in which the chloride ligands are replaced by an embedding potential reproduces rather well the spectral profile obtained for UO2Cl42-. Our results underscore the importance of the equatorial ligands to simulating core spectra at both uranium and oxygen edges.

Chemical bonding in americium oxides probed by X-ray spectroscopy

The electronic structure and the chemical state in Am binary oxides and Am-doped UO[Formula: see text] were studied by means of X-ray absorption spectroscopy at shallow Am core (4d and 5d) edges. In particular, the Am 5f states were probed and the nature of their bonding to the oxygen states was analyzed. The interpretation of the experimental data was supported by the Anderson impurity model (AIM) calculations which took into account the full multiplet structure due to the interaction between 5f electrons as well as the interaction with the core hole. The sensitivity of the branching ratio of the Am [Formula: see text] and [Formula: see text] X-ray absorption lines to the chemical state of Am was shown using Am binary oxides as reference systems. The observed ratio for Am-doped UO[Formula: see text] suggests that at least at low Am concentrations, americium is in the Am(III) state in the UO[Formula: see text] lattice. To confirm the validity of the applied AIM approach, the analysis of the Am 4f X-ray photoelectron spectra of AmO[Formula: see text] and Am[Formula: see text]O[Formula: see text] was also performed which revealed a good agreement between experiment and calculations. As a whole, AmO[Formula: see text] can be classified as the charge-transfer compound with the 5f occupancy ([Formula: see text]) equal to 5.73 electrons, while Am[Formula: see text]O[Formula: see text] is rather a Mott-Hubbard system with [Formula: see text] = 6.05.

Presence of uranium(V) during uranium(VI) reduction by Desulfosporosinus hippei DSM 8344T

Microbial U(VI) reduction influences uranium mobility in contaminated subsurface environments and can affect the disposal of high-level radioactive waste by transforming the water-soluble U(VI) to less mobile U(IV). The reduction of U(VI) by the sulfate-reducing bacterium Desulfosporosinus hippei DSM 8344^T, a close phylogenetic relative to naturally occurring microorganism present in clay rock and bentonite, was investigated. D. hippei DSM 8344^T showed a relatively fast removal of uranium from the supernatants in artificial Opalinus Clay pore water, but no removal in 30 mM bicarbonate solution. Combined speciation calculations and luminescence spectroscopic investigations showed the dependence of U(VI) reduction on the initial U(VI) species. Scanning transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy showed uranium-containing aggregates on the cell surface and some membrane vesicles. By combining different spectroscopic techniques, including UV/Vis spectroscopy, as well as uranium M₄-edge X-ray absorption near-edge structure recorded in high-energy-resolution fluorescence-detection mode and extended X-ray absorption fine structure analysis, the partial reduction of U(VI) could be verified, whereby the formed U(IV) product has an unknown structure. Furthermore, the U M₄ HERFD-XANES showed the presence of U(V) during the process. These findings offer new insights into U(VI) reduction by sulfate-reducing bacteria and contribute to a comprehensive safety concept for a repository for high-level radioactive waste.

Fast Chemical Contrast by X-ray Fluorescence Intensity Ratio Detection

We describe a protocol for efficient detection of the chemical state of an element based on X-ray emission (fluorescence) spectroscopy using a Bragg optics spectrometer. The ratio of intensities at two appropriately chosen X-ray emission energies is a self-normalized quantity largely free of experimental artifacts and can thus be recorded with high accuracy. As the X-ray fluorescence lines are chemically sensitive, the intensity ratio identifies the chemical state. Differences between chemical states in spatially inhomogeneous or temporally evolving samples can be identified already with low numbers of photon events. This reduces the time required for data acquisition by 2 orders of magnitude as compared to recording a full spectrum.

Deconvoluting Cr states in Cr-doped UO2 nuclear fuels via bulk and single crystal spectroscopic studies

Cr-doped UO₂ is a leading accident tolerant nuclear fuel where the complexity of Cr chemical states in the bulk material has prevented acquisition of an unequivocal understanding of the redox chemistry and mechanism for incorporation of Cr in the UO₂ matrix. To resolve this, we have used electron paramagnetic resonance, high energy resolution fluorescence detection X-ray absorption near energy structure and extended X-ray absorption fine structure spectroscopic measurements to examine Cr-doped UO₂ single crystal grains and bulk material. Ambient condition measurements of the single crystal grains, which have been mechanically extracted from bulk material, indicated Cr is incorporated substitutionally for U⁺⁴ in the fluorite lattice as Cr⁺³ with formation of additional oxygen vacancies. Bulk material measurements reveal the complexity of Cr states, where metallic Cr (Cr⁰) and oxide related Cr⁺² and Cr⁺³₂O₃ were identified and attributed to grain boundary species and precipitates, with concurrent (Cr⁺³_xU⁺⁴_1-x)O_2-0.5x lattice matrix incorporation. The deconvolution of chemical states via crystal vs. powder measurements enables the understanding of discrepancies in literature whilst providing valuable direction for safe continued use of Cr-doped UO₂ fuels for nuclear energy generation.

Cr2+ solid solution in UO2 evidenced by advanced spectroscopy

Advanced Cr-doped UO₂ fuels are essential for driving safe and efficient generation of nuclear energy. Although widely deployed, little is known about their fundamental chemistry, which is a critical gap for development of new fuel materials and radioactive waste management strategies. Utilising an original approach, we directly evidence the chemistry of Cr⁽³⁺⁾₂O₃-doped U⁽⁴⁺⁾O₂. Advanced high-flux, high-spectral purity X-ray absorption spectroscopy (XAS), corroborated by diffraction, Raman spectroscopy and high energy resolved fluorescence detection-XAS, is used to establish that Cr²⁺ directly substitutes for U⁴⁺, accompanied by U⁵⁺ and oxygen vacancy charge compensation. Extension of the analysis to heat-treated simulant nuclear fuel reveals a mixed Cr^2+/3+ oxidation state, with Cr in more than one physical form, explaining the substantial discrepancies that exist in the literature. Successful demonstration of this analytical advance, and the scientific underpinning it provides, opens opportunities for an expansion in the range of dopants utilised in advanced UO₂ fuels.

Synthesis and multi-scale properties of PuO2 nanoparticles: recent advances and open questions

Due to the increased attention given to actinide nanomaterials, the question of their structure-property relationship is on the spotlight of recent publications. Plutonium oxide (PuO₂) particularly plays a central role in nuclear energetics and a comprehensive knowledge about its properties when nanosizing is of paramount interest to understand its behaviour in environmental migration schemes but also for the development of advanced nuclear energy systems underway. The element plutonium further stimulates the curiosity of scientists due to the unique physical and chemical properties it exhibits around the periodic table. PuO₂ crystallizes in the fluorite structure of the face-centered cubic system for which the properties can be significantly affected when shrinking. Identifying the formation mechanism of PuO₂ nanoparticles, their related atomic, electronic and crystalline structures, and their reactivity in addition to their nanoscale properties, appears to be a fascinating and challenging ongoing topic, whose recent advances are discussed in this review.

Exact two-component Hamiltonians for relativistic quantum chemistry: Two-electron picture-change corrections made simple

Based on self-consistent field (SCF) atomic mean-field (amf) quantities, we present two simple yet computationally efficient and numerically accurate matrix-algebraic approaches to correct both scalar-relativistic and spin-orbit two-electron picture-change effects (PCEs) arising within an exact two-component (X2C) Hamiltonian framework. Both approaches, dubbed amfX2C and e(xtended)amfX2C, allow us to uniquely tailor PCE corrections to mean-field models, viz. Hartree-Fock or Kohn-Sham DFT, in the latter case also avoiding the need for a point-wise calculation of exchange-correlation PCE corrections. We assess the numerical performance of these PCE correction models on spinor energies of group 18 (closed-shell) and group 16 (open-shell) diatomic molecules, achieving a consistent ≈10^-5 Hartree accuracy compared to reference four-component data. Additional tests include SCF calculations of molecular properties such as absolute contact density and contact density shifts in copernicium fluoride compounds (CnF_n, n = 2,4,6), as well as equation-of-motion coupled-cluster calculations of x-ray core-ionization energies of 5d- and 6d-containing molecules, where we observe an excellent agreement with reference data. To conclude, we are confident that our (e)amfX2C PCE correction models constitute a fundamental milestone toward a universal and reliable relativistic two-component quantum-chemical approach, maintaining the accuracy of the parent four-component one at a fraction of its computational cost.

Ab Initio Prediction of Excited-State and Polaron Effects in Transient XUV Measurements of α-Fe2O3

Transient X-ray and extreme ultraviolet (XUV) spectroscopies have become invaluable tools for studying photoexcited dynamics due to their sensitivity to carrier occupations and local chemical or structural changes. One of the most studied materials using transient XUV spectroscopy is α-Fe₂O₃ because of its rich photoexcited dynamics, including small polaron formation. The interpretation of carrier and polaron effects in α-Fe₂O₃ is currently carried out using a semi-empirical method that is not transferrable to most materials. Here, an ab initio, Bethe-Salpeter equation (BSE) approach is developed that can incorporate photoexcited-state effects into arbitrary material systems. The accuracy of this approach is proven by calculating the XUV absorption spectra for the ground, photoexcited, and polaron states of α-Fe₂O₃. Furthermore, the theoretical approach allows for the projection of the core-valence excitons and different components of the X-ray transition Hamiltonian onto the band structure, providing new insights into old measurements. From this information, a physical intuition about the origins and nature of the transient XUV spectra can be built. A route to extracting electron and hole energies is even shown possible for highly angular momentum split XUV peaks. This method is easily generalized to K, L, M, and N edges to provide a general approach for analyzing transient X-ray absorption or reflection data.

To form or not to form: PuO2 nanoparticles at acidic pH

The aim of this study is to synthesize PuO2 nanoparticles (NPs) at low pH values and characterize the materials using laboratory and synchrotron-based methods. Properties of the PuO2 NPs formed under acidic conditions (pH 1-4) are explored here at the atomic scale. High-resolution transmission electron microscopy (HRTEM) is applied to characterize the crystallinity, morphology and size of the particles. It is found that 2 nm crystalline NPs are formed with a PuO2 crystal structure. High energy resolution fluorescence detected (HERFD) X-ray absorption spectroscopy at the Pu M4 edge has been used to identify the Pu oxidation states and recorded data are analysed using the theory based on the Anderson impurity model (AIM). The experimental data obtained on NPs show that the Pu(iv) oxidation state dominates in all NPs formed at pH 1-4. However, the suspension at pH 1 demonstrates the presence of Pu(iii) and Pu(vi) in addition to the Pu(iv), which is associated with redox dissolution of PuO2 NPs under acidic conditions. We discuss in detail the mechanism that affects the PuO2 NPs synthesis under acidic conditions and compare it with one in neutral and alkaline conditions. Hence, the results shown here, together with the first Pu M4 HERFD data on PuF3 and PuF4 compounds, are significant for the colloid facilitated transport governing the migration of plutonium in a subsurface environment.

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

Reaction of the N‐heterocylic carbene ligand ⁱPrIm (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.