Thorium dioxide (ThO 2 ) exhibits exceptional thermodynamic and redox stability, along with pronounced chemical inertness, owing to strong Th-O bonding and the high stability of Th 4+ . These properties make it a...

Cr-doped UO₂ fuels are increasingly adopted for their superior in-reactor performance compared to undoped UO₂, but their spent fuel behaviour, particularly potential Cr speciation and fission product reactivity, remains poorly understood. This investigation has used high energy resolution fluorescence detected X-ray absorption near edge structure (HERFD-XANES) spectroscopy to examine speciation of Cr and Pr/Gd within 200 ppm Cr-doped (U4.4+0.7Pr3+0.3)O2-x and 200 ppm Cr-doped (U4.4+0.7Gd3+0.3)O2-x compounds. Despite both being UO2 soluble and undersaturated, analysis indicates that Cr3+ and Pr3+/Gd3+ form perovskite type (Pr3+/Gd3+)Cr3+O3 phases, consistent with classical “grey phases” of spent fuel. The radiation tolerance of these phases was examined via swift heavy ion irradiations of PrCrO3 and GdCrO3 compounds where electron microscopy and grazing incidence synchrotron diffraction indicate significant amorphization but retention of the crystal structure. The investigation highlights the pertinence of considering the chemistry of dopants used for nuclear fuel enhancements regarding their speciation during irradiation and subsequent occurrence within spent fuel.

Single crystals ofCsTb(CrO4)2 andCsDy(CrO4)2, where the lanthanide metals arein the trivalentstate under ambient conditions, have been investigated under high-pressureconditions on the gigapascal scale utilizing a diamond anvil cell.These compounds were characterized by single-crystal X-ray diffractionin addition to high-pressure solid-state UV-vis-NIR spectroscopy,Raman spectroscopy, and Tb L3-edge high-energy-resolutionfluorescence-detected X-ray absorption near-edge structure (HERFD-XANES).The high-pressure UV-vis-NIR spectra reveal strong broadening of themetal-to-ligand charge transfer band to lower energies, associatedwith a visible color change from yellow to dark red/black. Clear evidencefor the stabilization of Tb4+ under high pressure is providedby the appearance of a second edge feature characteristic of Tb4+, starting at 19.62 GPa in the high-pressure L3-edge HERFD-XANES at around 7528 eV. This represents the first exampleof Tb4+ being stabilized by high pressure and expands uponthe limited chemistry of terbium in the tetravalent state.

Abstract Uranium contamination is a major global concern, as its chemical and radiological effects threaten ecosystems and human health, particularly in mining-impacted regions. Pentavalent uranium is a key but often overlooked intermediate in U biogeochemistry, challenging the conventional view of a direct U(VI) to U(IV) transition and precipitation in natural waters. Here we demonstrate the formation and stability of U(V) under environmentally relevant mine-water conditions using advanced spectroscopic and microscopic analyses. Our data reveal concurrent U(VI) reduction to U(IV), as biogenic uraninite nanoparticles, and to U(V) as FeU(V)O₄ nanoparticles and U(V)-carbonate complexes. U(V) persists for at least 130 days under anoxic conditions and four weeks after exposure to oxygen. Microbial community analysis reveals enrichment of fermentative and sulphate-reducing taxa, consistent with redox conditions favouring U reduction. By demonstrating the stability of immobilised U(V) alongside U(IV), this work advances the understanding of uranium biogeochemistry and offers new insights for sustainable remediation strategies.

In the context of spent fuel recycling and the valorization of plutonium, (U,Pu)O2 mixed oxides (MOX) have been developed for use in French Pressurized Water Reactors (PWR). They are also leading candidates for some GEN IV reactor concepts, such as sodium-cooled fast reactors (SFR). One of the critical challenges in the nuclear industry is the mastery of the nuclear fuel cycle, specifically plutonium multirecycling. In order to achieve this goal, it is crucial to identify the secondary phases created during irradiation. In this work, (U,Pu)O2 MOX have been doped with 11 stable fission products (FP) (Sr, Y, La, Nd, Ce, Zr, Mo, Pd, Rh, Ru, Ba) to reproduce FP-based precipitates existing in the real spent fuel. The structural and microstructural properties of these secondary phases were characterized by coupling scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS), Electron Probe MicroAnalysis (EPMA), and synchrotron techniques such as X-ray Absorption Spectroscopy (XAS) and Synchrotron Powder X-ray Diffraction (SP-XRD). This analysis highlights the relationship between the partial segregation among metallic FP (Mo, Pd, Rh, Ru) and their crystallographic structures, as well as the speciation shift of several FP induced by the addition of Ba. The synthesized SIMMOX samples present a secondary phase representative of irradiated MOX and can be used as an effective model material to study spent nuclear fuel and its reprocessing.

Abstract Actinide systems continue to raise many unanswered questions on topics involving the number of electrons in valence states, the degree of f-electron localization, and the character of their chemical bonding. Their partially filled 6d and 5f valence shells are responsible for most of their complex behaviour. Understanding the intricate electronic structure of actinides requires the use of advanced experimental and theoretical techniques. Among the experimental approaches, resonant inelastic x-ray scattering and x-ray absorption near edge structure in the high energy resolution fluorescence detection mode at the actinide M 4,5 edges have proven to be powerful techniques for investigating their electronic structure. Here, we review the fundamentals of these x-ray spectroscopies and the theoretical advances in electronic structure calculations using data recorded on 5f electron systems at the An M 4,5 edges (An = Th, U, Np, Pu).

Schwertmannite and jarosite are naturally occurring iron (Fe) oxyhydroxysulfates with strong sorption capacities for hexavalent uranium [U(VI)] in various acidic sulfate-rich environments. These metastable minerals commonly undergo recrystallization, particularly in the presence of dissolved Fe2+ [Fe(II)(aq)], which may influence the fate of associated U(VI). Here, we quantified molecular-level changes in U repartitioning and speciation when U(VI)-sorbed schwertmannite and jarosite reacted with Fe(II)(aq) under near-neutral and anaerobic conditions over 2 weeks. The results show that Fe(II)(aq) additions promoted rapid mineral transformation to goethite via a dissolution-reprecipitation pathway, proceeding (near-completely) for schwertmannite but slowly and incompletely for jarosite. Importantly, even at early transformation stages when goethite likely only started forming on the surface of the transforming minerals, the recrystallization process led to near-complete retention of U, predominantly as U(VI), within the structure of the neo-formed goethite. Subsequent U reduction to U(V) increased with time but remained incomplete, even after extensive mineral transformation in the presence of 1-50 mM Fe(II)(aq) for 2 weeks. The results demonstrate that Fe(II)-promoted recrystallization of Fe-oxyhydroxysulfates can rapidly and persistently lock both U(VI) and U(V) into chemically stable goethite, with important implications for predicting U behavior and designing remediation strategies in various acidic and U-contaminated environments.

We report X-ray absorption spectroscopy (XAS) studies of tetravalent berkelium (Bk). Chelation of BkIII by siderophore-inspired chelators stabilized BkIV under mild conditions, as confirmed by absorption and luminescence spectroscopies and supported by surrogate experiments with the lanthanide counterpart to Bk, cerium (Ce). The Bk complexes remained, at least partially, in the tetravalent oxidation state under the high-energy X-ray beam despite previous attempts that had resulted in immediate in situ beam reduction, owing to cryogenic measurements on solid-state complexes. Computational calculations matched the experimental data, and BkIV-O interatomic distances were determined by extended X-ray absorption fine structure measurements (EXAFS) to gain understanding into BkIV coordination behavior with organic ligands. These results pave the way for future studies of BkIV and other transplutonium compounds to investigate electronic structure and complexation behavior across the 5f series.

Actinide systems continue to raise many unanswered questions on topics involving the number of electrons in valence states, the degree of f-electron localization, and the character of their chemical bonding. Their partially filled 6d and 5f valence shells are responsible for most of their complex behaviour. Understanding the intricate electronic structure of actinides requires the use of advanced experimental and theoretical techniques. Among the experimental approaches, resonant inelastic x-ray scattering and x-ray absorption near edge structure in the high energy resolution fluorescence detection mode at the actinide M4,5edges have proven to be powerful techniques for investigating their electronic structure. Here, we review the fundamentals of these x-ray spectroscopies and the theoretical advances in electronic structure calculations using data recorded on 5f electron systems at the An M4,5edges (An = Th, U, Np, Pu).

Abstract Two series of actinide tetrachloride complexes with the monodentate carboxylic amide ligands phthalimidine (H-phthal) and 2-pyridone (HPyO) of the general formula [AnIVCl4(L)2] (An = U, Np, Pu) were synthesized and characterized in the solid state and in solution. SC-XRD analysis revealed an octahedral coordination environment with trans orientation of either H-phthal or HPyO in all cases, with and without THF molecules hydrogen-bonded to the NH group of the ligands. In the case of [NpCl4(H-phthal)2], the absence of THF leads to a significant distortion of the coordination geometry around the metal, including the shortest Np–Cl bonds observed so far (2.4394(8) Å). A quantum chemical bonding analysis (QTAIM) showed that the main bonding interaction is found between the actinide and chloride ligands, with the amides rather loosely bound. Despite their unusually low coordination number, the complexes were found to be stable, even in coordinating solvents. This allowed for an in-depth analysis of their 1H and 13C NMR spectroscopy. Due to their pseudoD4h symmetry in the first coordination sphere, these complexes are excellent model systems to analyze their electronic structure and magnetic properties. We applied SQUID magnetometry to extract magnetic properties (μeff) and crystal field parameters ( Bqk, Wybourne) in combination with quantum chemical calculations, which allows for the comprehensive description of the electronic ground state from an experimental and theoretical point of view.

Actinide nanoparticles (NPs) are widely recognized for their role as a potentially highly mobile form of radioactive contaminants in the environment. In recent years, research has increasingly focused on elucidating their formation mechanisms, atomic structure, and physicochemical properties. The application of synchrotron radiation techniques is central to the detailed characterization of their atomic structure and oxidation state. This review retraces the evolution of actinide NPs research and highlights recent achievements enabled by high‐energy‐resolution fluorescence‐detected X‐ray absorption near edge structure, used in correlation with complementary synchrotron‐based methods.

The complexity of actinide chemistry and physics, driven by intricate electronic structures, variable oxidation states, and radioactive properties, poses significant challenges for scientific exploration. Synchrotron radiation methods, including X‐ray Absorption Spectroscopy (XAS), X‐ray Emission Spectroscopy (XES), high energy resolution fluorescence detection (HERFD) XAS, resonant inelastic X‐ray scattering (RIXS) and X‐ray Diffraction (XRD), have proven to be transformative tools in addressing these challenges. These advanced methods enable detailed investigations of local environments, oxidation states, and phase transitions, offering critical insights into nuclear fuel management, environmental remediation, and the development of advanced materials. This work highlights the developments and applications of synchrotron‐based methods and their analysis for studying actinide systems at the Rossendorf beamline at the ESRF (Grenoble, France). The results underscore the pivotal role of the combination of synchrotron techniques and advanced theoretical modeling to unravel the complexities of actinide materials.

In this study, the influence of the working atmosphere on the sinterability and chemical durability of Nd-doped UO2 mixed oxides was investigated. To this end, the starting powder was first prepared by a hydroxide coprecipitation route, resulting in a nano-sized granulometry combined with a high specific surface area. The powders were then converted to oxides by heating and sintered in pellet form at 1600 degrees C under an argon or reducing (Ar-4 %H2) atmosphere. The use of argon or reducing atmosphere resulted in very different densification pathways and final microstructures. The reducing sintering atmosphere hindered the uranium (IV) oxidation that could occur at high temperature, leading to the formation of U3O8, as was the case when working under argon atmosphere. Regarding the microstructure of the sintered pellets, the use of an argon sintering atmosphere resulted in an average grain size ten times larger than that of a reducing sintering atmosphere, while macroscopic properties such as relative density, porosity and homogeneity of cation distribution at the pellet scale remained the same. Nevertheless, a slight local enrichment of Nd at the grain boundaries was observed for the pellet sintered under Ar-4 %H2. In a second step, the study of the chemical durability of these sintered samples showed a significant influence of the sintering atmosphere on the dissolution kinetics and mechanism. These differences could be related to the microstructural properties of the pellets, i.e. the average grain size and the occurrence of grain boundaries. The cation distribution in the pellets could also influence their chemical durability, such as local Nd enrichment, the formation of defects in the oxygen sublattice and the presence of a different fraction of U(V) depending on the sintering atmosphere, as shown by HERFD-XANES measurements. The use of reducing or argon sintering atmospheres could even direct the charge compensation mechanisms that occur into the solid, thereby simultaneously affecting the sinterability and chemical durability of the samples.

Complexes of tetravalent actinides (An: Th, U, Np, and Pu) with the bidentate (N,S)-donor ligand pyridine-2-thiolate (2-PyS, PyS-) were synthesized in 1:4 or 1:5 ratios. This includes the first structurally characterized Np complex with (N,S)-donor ligands, filling a notable gap in the An coordination chemistry. An improved synthetic approach with PyS-SiMe3 enabled efficient formation of the 1:4 complexes in THF as a coordinating solvent. The compounds were comprehensively characterized in solution and in the solid phase, supported by quantum chemical calculations. Experimental and theoretical results show matching trends in the binding behavior of AnIV. The covalent bond contributions in An-N and An-S bonding increase along the series of An from Th to Pu. The bonds to the soft sulfur donors consistently have the highest covalent contributions with a remarkably high percentage for Pu-S (IQA analysis: >34%) over the harder N donors or An-O bonds of coordinating THF. High-resolution X-ray absorption and infrared spectroscopy indicate similar electronic and structural properties across AnIV complexes, while SQUID magnetometry uncovered significant differences in magnetic behavior at low temperatures depending on the complex compositions. This work advances the understanding of An-ligand bonding, emphasizing covalency and electronic structures, and expands fundamental insights into An chemistry.

Understanding the redox transitions that control rhenium geochemistry is central to paleoredox and geochronology studies, as well as predicting the fate of chemically similar hazardous oxyanions in the environment such as pertechnetate. However, detailed mechanistic information regarding rhenium redox transitions in anoxic systems is scarce. Here, we performed a comprehensive laboratory study of rhenium redox transitions on variably oxidized magnetite nanoparticle surfaces. Through high-end spectroscopic and microscopic tools, we propose an abiotic transition pathway in which aqueous iron(II) ions in the presence of pure or preoxidized magnetite serve as an electron source to reduce rhenium(VII) to individual rhenium(IV) atoms or small polynuclear species on nanoparticle surfaces. Notably, iron(II) ions recharged preoxidized magnetite nanoparticles exhibit a maghemite core and a magnetite shell, challenging the traditional core-shell magnetite-maghemite model. This study provides a fundamental understanding of redox processes governing rhenium fate and transport in the environment and enables an improved basis for predicting its speciation in geochemical systems.

Yb5Rh6Sn18 crystallizes with a unique structural arrangement [space group P42/nmc, a = 9.6997(4) & Aring;, c = 13.7710(7) & Aring;], which is related with primitive cubic Yb3Rh4Sn13 and body-centered tetragonal (Sn1-xTbx)Tb4Rh6Sn18 types. X-ray absorption spectroscopy showed that Yb atoms exhibit temperature-dependent valence fluctuations (VF) (i.e., intermediate valence state). Its complex mechanism is corroborated by the fact that the well-pronounced maximum in magnetic susceptibility can only be fairly described by the Bickers-Cox-Wilkins model developed for a J = 3/2 multiplet, atypical for Yb ions. Both Hall and Seebeck coefficients revealed a switch of the sign, indicating the change of charge carrier type from electrons to holes between 120 and 220 K. Both these effects together with electrical resistivity and theoretical DFT calculations confirm Yb5Rh6Sn18 to be a metal, which disobeys the free electron gas theory. 'Rattling' motion of Sn1 atoms within the enlarged 16-vertices distorted Frank-Kasper polyhedra, concluded from the specific heat measurements, is argued to be the main reason for the appearance of a phonon resonance behavior, resulting in an ultra-low thermal conductivity in the studied stannide.

X-ray spectroscopy techniques are critical in the electronic structure analysis of high-valent lanthanides. The interpretation of multipeaked features at the lanthanide L3-edge has remained a challenging question, as it is observed across a range of material classes. A series of structurally related Ce4+complexes were prepared to probe the potential ligand field perturbation of the ground state within the series. The tuning of relative 4f and ligand orbital energies in homoleptic and heteroleptic tetravalent Ce imidophosphorane complexes is achieved through ligand derivatization and is clearly demonstrated by UV-vis spectroscopy and electrochemically measured redox potentials. However, Ce L3-edge high-energy resolution fluorescence-detected (HERFD) X-ray absorption near-edge structure (XANES) spectra present features at consistent numbers and energies across the range of complexes. Resonant inelastic X-ray scattering (RIXS) is employed to visualize the observed features in the HERFD-XANES spectra. Large complete active space configuration interaction singles and doubles (CASCISD) calculations demonstrate that the ground-state wave function of all complexes can be described employing a single determinant. As a result, the multielectron feature at the L3-edge observed in this study for the Ce4+ imidophosphorane complexes is described as excited-state multiconfigurational behavior that is independent of ligand variation, in systems where the ground state is described using the simple single determinant wave function.

The ZrO2-CeO2 system is fundamental to various technological applications, yet unresolved questions persist regarding cation miscibility and the occurrence of metastable phases in the Zr1-xCexO2 phase diagram. This work addresses these gaps through a comprehensive investigation of Zr1-xCexO2 compositions with varying cerium concentrations and incorporating Eu3+ as a luminescent probe. Synchrotron powder X-ray diffraction analysis unveiled a miscibility gap between 20 and 50 mol % cerium. Beyond this gap, the formation of solid solutions and multiple crystalline phases was observed, including tetragonal prime (t') and tetragonal double prime (t″) structures, depending on cerium content. Raman investigations revealed a unique distortion band in all compositions containing the t' phase. Our high energy resolution fluorescence detected X-ray absorption near edge structure spectroscopy (HERFD-XANES) analysis implies that this feature results from oxygen ion displacement in the t' structure. Luminescence spectroscopy of the europium environment revealed distinct excitation and emission spectra across the various crystal phases, enabling unambiguous identification of all metastable phases. These findings highlight the complex polymorphism of the ZrO2-CeO2 system. The ability to precisely control phase composition offers significant potential for optimizing properties for diverse applications, including oxygen sensors, three-way catalysts, and solid oxide fuel cells for clean, sustainable energy generation.

In the marine environment, hexavalent uranium, U6+, is incorporated into primary carbonate minerals with the same isotopic composition (delta U-238) as the seawater in which they are formed. Yet, modern marine carbonate sediments carry heavier U isotope compositions. This enrichment of heavy U isotopes has been linked to biogenic U reduction in and below the Fe-reducing zone inside the sediment. Still, the oxidation state(s) of uranium in marine carbonate sediments undergoing syndepositional diagenesis has never been measured before. Here, we 1) present an anaerobic ion chromatographic technique based on the TEVA (R) resin to chemically separate and quantify abundances of tetravalent U4+ and hexavalent U6+ fractions in the carbonate, and 2) compare the results from ion chromatography to U L3 edge HERFD-XANES spectroscopic measurements of the total U in sediments to 3) estimate U oxidation states of fresh carbonate sediments from a modern seawater-fed lake and ancient limestones. We find that our anaerobic extraction technique can provide credible evaluations of reduced U4+ and oxidized U6+ contents, applicable to carbonate sediments and rocks. Our results show that U resides both in reduced and oxidized states in modern carbonate sediments and ancient carbonate rocks. By comparing air-exposed, oven-dried samples to samples always kept under strictly anaerobic condition, we find that the majority of authigenic U in modern carbonate sediments resides in oxidation-sensitive phases that accumulate with sediment depth, instead of being structurally bound in carbonate minerals (aragonite and calcite). We propose a model to account for the observed trends in U oxidation state, U phase associations, and U isotope fractionation, where a substantial fraction of U in the sediments is likely delivered via microbial reduction and precipitated as a non-crystalline, reduced form near the sediment-water interface. We suggest these oxidation-sensitive reduced U species participate in redox cycling where some U is re-oxidized and perhaps bio-reduced again later, for example in the presence of Fe(III) mineral surfaces that undergo reductive dissolution with depth. Simultaneously, a continued incorporation of recalcitrant and isotopically light (i.e. U-238-depleted) U from the pore fluids into diagenetic carbonate may occur. The determination of U oxidation states in modern carbonates in this study helps to bridge a gap in our knowledge of how U isotope signals are affected by syn-sedimentary diagenetic U transformations, opening new avenues for understanding sedimentary U cycling and improving the delta U-238 paleo redox proxy.