Welcoming a new Main Editor of Journal of Synchrotron Radiation.

Introducing a new Main Editor of JSR.

Electronic Structure Interpretation: How much do we understand the Ce L3 XANES?

Historically, cerium has been attractive for pharmaceutical and industrial applications. The cerium atom has the unique ability to cycle between two chemical states (Ce(III) and Ce(IV)) and drastically adjust its electronic configuration: [Xe] 4f1 5d1 6s2 in response to a chemical reaction. Understanding how electrons drive chemical reactions is an important topic. The most direct way of probing the chemical and electronic structure of materials is by X-ray absorption spectroscopy (XAS) or X-ray absorption near-edge structure (XANES) in high energy resolution fluorescence detection (HERFD) mode. Such measurements at the Ce L3 edge have the advantage of a high penetration depth, enabling in-situ reaction studies in a time-resolved manner and investigation of material production or material performance under specific conditions. But how much do we understand Ce L3 XANES? This article provides an overview of the information that can be extracted from experimental Ce L3 XAS/XANES/HERFD data. A collection of XANES data recorded on various cerium systems in HERFD mode is presented here together with detailed discussions on data analysis and the current status of spectral interpretation, including electronic structure calculations.

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.

Application of multi-edge HERFD-XAS to assess the uranium valence electronic structure in potassium uranate (KUO3).

The uranium valence electronic structure in the prototypical undistorted perovskite KUO3 is reported on the basis of a comprehensive experimental study using multi-edge HERFD-XAS and relativistic quantum chemistry calculations based on density functional theory. Very good agreement is obtained between theory and experiments, including the confirmation of previously reported Laporte forbidden f-f transitions and X-ray photoelectron spectroscopic measurements. Many spectral features are clearly identified in the probed U-f, U-p and U-d states and the contribution of the O-p states in those features could be assessed. The octahedral crystal field strength, 10Dq, was found to be 6.6 (1.5) eV and 6.9 (4) eV from experiment and calculations, respectively. Calculated electron binding energies down to U-4f states are also reported.

Effective coordination numbers from EXAFS: general approaches for lanthanide and actinide dioxides.

Extended X-ray absorption fine structure (EXAFS) is a comprehensive and usable method for characterizing the structures of various materials, including radioactive and nuclear materials. Unceasing discussions about the interpretation of EXAFS results for actinide nanoparticles (NPs) or colloids were still present during the last decade. In this study, new experimental data for PuO2 and CeO2 NPs with different average sizes were compared with published data on AnO2 NPs that highlight the best fit and interpretation of the structural data. In terms of the structure, PuO2, CeO2, ThO2, and UO2 NPs exhibit similar behaviors. Only ThO2 NPs have a more disordered and even partly amorphous structure, which results in EXAFS characteristics. The proposed new core-shell model for NPs with calculated effective coordination number perfectly fits the results of the variations in a metal-metal shell with a decrease in NP size.

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.

Biomineralization of Uranium-Phosphates Fueled by Microbial Degradation of Isosaccharinic Acid (ISA).

Geological disposal is the globally preferred long-term solution for higher activity radioactive wastes (HAW) including intermediate level waste (ILW). In a cementitious disposal system, cellulosic waste items present in ILW may undergo alkaline hydrolysis, producing significant quantities of isosaccharinic acid (ISA), a chelating agent for radionuclides. Although microbial degradation of ISA has been demonstrated, its impact upon the fate of radionuclides in a geological disposal facility (GDF) is a topic of ongoing research. This study investigates the fate of U(VI) in pH-neutral, anoxic, microbial enrichment cultures, approaching conditions similar to the far field of a GDF, containing ISA as the sole carbon source, and elevated phosphate concentrations, incubated both (i) under fermentation and (ii) Fe(III)-reducing conditions. In the ISA-fermentation experiment, U(VI) was precipitated as insoluble U(VI)-phosphates, whereas under Fe(III)-reducing conditions, the majority of the uranium was precipitated as reduced U(IV)-phosphates, presumably formed via enzymatic reduction mediated by metal-reducing bacteria, including Geobacter species. Overall, this suggests the establishment of a microbially mediated "bio-barrier" extending into the far field geosphere surrounding a GDF is possible and this biobarrier has the potential to evolve in response to GDF evolution and can have a controlling impact on the fate of radionuclides.

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.

Size Dependence of Lattice Parameter and Electronic Structure in CeO2 Nanoparticles.

Intrinsic properties of a compound (e.g., electronic structure, crystallographic structure, optical and magnetic properties) define notably its chemical and physical behavior. In the case of nanomaterials, these fundamental properties depend on the occurrence of quantum mechanical size effects and on the considerable increase of the surface to bulk ratio. Here, we explore the size dependence of both crystal and electronic properties of CeO2 nanoparticles (NPs) with different sizes by state-of-the art spectroscopic techniques. X-ray diffraction, X-ray photoelectron spectroscopy, and high-energy resolution fluorescence-detection hard X-ray absorption near-edge structure (HERFD-XANES) spectroscopy demonstrate that the as-synthesized NPs crystallize in the fluorite structure and they are predominantly composed of CeIV ions. The strong dependence of the lattice parameter with the NPs size was attributed to the presence of adsorbed species at the NPs surface thanks to Fourier transform infrared spectroscopy and thermogravimetric analysis measurements. In addition, the size dependence of the t2g states in the Ce LIII XANES spectra was experimentally observed by HERFD-XANES and confirmed by theoretical calculations.

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.