In Situ High-Temperature Raman Spectroscopy of UCl3: A Combined Experimental and Theoretical Study.

Uranium trichloride (UCl3) has received growing interest for its use in uranium-fueled molten salt reactors and in the pyrochemical processing of used fuel. In this paper, we report for the first time the experimentally determined Raman spectra of UCl3, at both ambient condition and in situ high temperatures up to 871 K. The frequencies of five of the Raman-active vibrational modes (vi) of UCl3 exhibit a negative temperature derivative ((∂νi/∂T)P) with increasing temperature. This red-shift behavior is likely due to the elongation of U-Cl bonds. The average isobaric mode Grüneisen parameter (γiP = 0.91 ± 0.02) of UCl3 was determined through use of the coefficient of thermal expansion published in Vogel et al. (2021) and the (∂νi/∂T)P values determined in this study. These results are in general agreement with those calculated here by density functional theory (DFT+U). Finally, a comparison of the ambient band positions of UCl3 to those of isostructural lanthanide (La-Eu) and actinide chlorides (Am-Cf) has been made.

Trace Impurities Identified as Forensic Signatures in CMX-5 Fuel Pellets Using X-ray Spectroscopic Techniques.

Small-particle analysis is a highly promising emerging forensic tool for analysis of interdicted special nuclear materials. Integration of microstructural, morphological, compositional, and molecular impurity signatures could provide significant advancements in forensic capabilities. We have applied rapid, high-sensitivity, hard X-ray synchrotron chemical imaging to analyze impurity signatures in two differently fabricated fuel pellets from the 5th Collaborative Materials Exercise (CMX5) of the IAEA Nuclear Forensics International Working Group. The spatial distributions, chemical compositions, and morphological and molecular characteristics of impurities were evaluated using X-ray absorption near-edge structure (XANES) and X-ray fluorescence chemical imaging to discover principal impurities, their granularity, particle sizes, modes of occurrence (distinct grains vs incorporation in the UO2 lattice), and sources and mechanisms of incorporation. Differences in UO2+x stoichiometry were detected at the microscale in nominally identical UO2 ceramics (CMX5-A and CMX5-B), implying the presence of multiple UO2 host phases with characteristic microstructures and feedstock compositions. Al, Fe, Ni, W, and Zr impurities and integrated impurity signature analysis identified distinctly different pellet synthesis and processing methods. For example, two different Al, W, and Zr populations in the CMX5-B sample indicated a more complex processing history than the CMX5-A sample. K-edge XANES measurements reveal both metallic and oxide forms of Fe and Ni but with different proportions between each sample. Altogether, these observations suggest multiple sources of impurities, including fabrication (e.g., force-sieving) and feedstock (mineral oxides). This study demonstrates the potential of synchrotron techniques to integrate different signatures across length scales (angstrom to micrometer) to detect and differentiate between contrasting UO2 fuel fabrication techniques.

Dynamics of Cation-Induced Conformational Changes in Nanometer-Sized Uranyl Peroxide Clusters.

Conformational changes of the pyrophosphate (Pp)-functionalized uranyl peroxide nanocluster [(UO2)24(O2)24(P2O7)12]48- ({U24Pp12}), dissolved as a Li/Na salt, can be induced by the titration of alkali cations into solution. The most symmetric conformer of the molecule has idealized octahedral (Oh) molecular symmetry. One-dimensional 31P NMR experiments provide direct evidence that both K+ and Rb+ ions trigger an Oh-to-D4h conformational change within {U24Pp12}. Variable-temperature 31P NMR experiments conducted on partially titrated {U24Pp12} systems show an effect on the rates; increased activation enthalpy and entropy for the D4h-to-Oh transition is observed in the presence of Rb+ compared to K+. Two-dimensional, exchange spectroscopy 31P NMR revealed that magnetization transfer links chemically unique Pp bridges that are present in the D4h conformation and that this magnetization transfer occurs via a conformational rearrangement mechanism as the bridges interconvert between two symmetries. The interconversion is triggered by the departure and reentry of K (or Rb) cations out of and into the cavity of the cluster. This rearrangement allows Pp bridges to interconvert without the need to break bonds. Cs ions exhibit unique interactions with {U24Pp12} clusters and cause only minor changes in the solution 31P NMR signatures, suggesting that Oh symmetry is conserved. Single-crystal X-ray diffraction measurements reveal that the mixed Li/Na/Cs salt adopts D2h molecular symmetry, implying that while solvated, this cluster is in equilibrium with a more symmetric form. These results highlight the unusually flexible nature of the actinide-based {U24Pp12} and its sensitivity to countercations in solution.

Supramolecular Assembly of Geometrically Unstable Hybrid Organic-Inorganic Uranyl Peroxide Cage Clusters and Their Transformations.

An aromatic ligand was introduced into the synthesis of a uranyl peroxide polyoxometalate formulated as K32(UO2)19(O2)26(OH)2(C6H4P2O6)4·65H2O that consists of a unique "open oyster" shaped structure (U19) with intramolecular H-bonds. In the solid state, K-π and π-π interactions as well as K-O bonds enable the formation of a supramolecular network between U19 clusters. U19 adopts an incomplete fullerene topology and was utilized as a precursor from which the geometrically favored U24 structure was produced. A potassium-encapsulated U24 structure was obtained upon heating the solution containing U19.

Uranyl-Peroxide Capsule Self-Assembly in Slow Motion.

Uranyl-peroxide capsules are the newest family of polyoxometalates. Although discovered 13 years previously with over 70 topologies reported, there is a lack in the fundamental understanding of assembly mechanisms, particularly the role of the alkali counterions. Herein, the reaction pathway and assembly of uranyl peroxide capsules is reported by tracking the conversion from K+ uranyl triperoxide monomer to the K+ uranyl-peroxide U28 capsule by means of small-angle X-ray scattering and Raman spectroscopy. For the first time, the K+ uranyl-peroxide pentamer face is isolated and structurally characterized, giving credence to the long-held belief that these geometric faces serve as building blocks to the fully formed capsules. Once isolated and re-dissolved, the pentamer face undergoes rapid conversion to capsule forms, underlining its high reactivity that challenges its isolation. Calorimetric measurements of the studied species confirms the pentamer lies on the energy landscape between the monomer and capsule.

Effects of H2O2 Concentration on Formation of Uranyl Peroxide Species Probed by Dissolution of Uranium Nitride and Uranium Dioxide.

Dissolution of uranium materials in alkaline aqueous conditions containing H2O2 results in uranyl peroxide species in solution, including anionic uranyl peroxide cage clusters. Uranyl peroxide cage clusters are generally highly soluble in water, where they persist as aqueous macroanions. Previous studies indicate that uranyl cluster speciation and dissolution of uranium materials is impacted by the concentration of alkali metal in solution, but in these studies, high concentrations of H2O2 were used. Herein, the role of hydrogen peroxide concentration is examined relative to the dissolution of powdered UN and UO2. Lower initial H2O2 concentrations reduce dissolution of UO2 and UN and tend to produce simple (small) uranyl peroxide species rather the highly soluble uranyl peroxide clusters. H2O2 availability will have implications for uranyl speciation and solubility where spent nuclear fuel is in contact with water and where alkaline peroxide conditions are used in dissolution of nuclear material.

Neptunyl Peroxide Chemistry: Synthesis and Spectroscopic Characterization of a Neptunyl Triperoxide Compound, Ca2[NpO2(O2)3]·9H2O.

Little is known about the crystal chemistry of neptunyl peroxide compounds compared to uranyl peroxide compounds, for which dozens of structures have been described. Uranyl peroxides are formed over a broad range of pH and solution conditions, but neptunyl peroxide chemistry is complicated by the ability of H2O2 to act as an oxidizing or reducing agent for Np, depending on the conditions present. The combination of Np(V) in 1 M HCl, H2O2, and CaCl2 under alkaline conditions leads to the immediate crystallization of a neptunyl triperoxide monomer, Ca2[NpO2(O2)3]·9H2O, which is the first Np(VI)-based peroxide compound to be characterized in the solid state and is isostructural to Ca2[UO2(O2)3]·9H2O. The crystal structure reveals bond distances of 1.842(7) Å that are the longest reported to date for nonbridging Np(VI)-Oyl bonds. Computational studies probe the oxidation state and bond distances of the monomer unit and differences in Raman spectra of the neptunyl and uranyl triperoxide compounds.

Energetic Trends in Monomer Building Blocks for Uranyl Peroxide Clusters.

The uranyl triperoxide anionic monomer is a fundamental building block for uranyl peroxide polyoxometalate capsules. The reaction pathway from the monomer to the capsule can be greatly altered by the counterion: both the reaction rate and the resulting capsule structure. We synthesized and characterized uranyl triperoxides Mg2UO2(O2)3·13H2O (MgUT), Ca2UO2(O2)3·9H2O (CaUT), Sr2UO2(O2)3·9H2O (SrUT), and K4UO2(O2)3·3H2O (KUT) and compared their thermodynamic stabilities. The enthalpies of formation from oxides and elements of these compounds were calculated by thermochemical cycles from measurements by high temperature oxide melt drop solution calorimetry. Their formation enthalpies from oxides become more negative linearly as a function of the increasing basicity of the respective oxides on the Smith scale. This relationship holds for previously Li and Na analogues. Further affirming the trend, Δ Hf,ox of MgUT departs from linearity, due to the distinct bonding environment of Mg2+, as compared to the other alkalis and alkaline earths in the series.

Mixed-Valent Cyanoplatinates Featuring Neptunyl-Neptunyl Cation-Cation Interactions.

The tetracyanoplatinate ligand was employed in synthesizing the first neptunyl cyanoplatinate complexes. Results indicate in situ oxidation of Pt(II) by Np(V/VI) to form mixed-valent Pt-Pt stacked columnar chains linked by cation-cation interaction induced chains of Np(V) polyhedra into a two-dimensional sheet structure. The Pt-Pt stacking distances of 3.04-3.05 Å are the longest reported columnar platinophilic interactions among mixed-valent tetracyanoplatinate structures. These complexes further illustrate the marked chemical differences and structural diversity of solid-state Np(V) coordination complexes with regard to Np(VI) and U(VI).

Charge Density Influence on Enthalpy of Formation of Uranyl Peroxide Cage Cluster Salts.

More than 60 unique uranyl peroxide cage clusters have been reported that contain as many as 124 uranyl ions and that have overall diameters extending to 4 nm. They self-assemble in water under ambient conditions, are models for understanding structure-size-property relations as well as testing computational models for actinides, and have potential applications in nuclear fuel cycles. High-temperature drop solution calorimetry has been used to derive the enthalpies of formation of the salts of seven topologically diverse uranyl peroxide cage clusters containing from 22 to 28 uranyl ions that are bridged by various combinations of peroxide, pyrophosphate, and phosphite. The enthalpies of formation of these seven salts, as well as three salts of other uranyl peroxide clusters reported earlier, are dominated by the interactions of the alkali countercations with the clusters. There is an approximately linear relationship between the enthalpies of formation of the cluster salts and the charge density of the corresponding uranyl peroxide cluster, wherein salts containing clusters with higher charge densities have more negative enthalpies of formation.

Complexity of Uranyl Peroxide Cluster Speciation from Alkali-Directed Oxidative Dissolution of Uranium Dioxide.

Solid UO2 dissolution and uranium speciation in aqueous solutions that promote formation of uranyl peroxide macroanions was examined, with a focus on the role of alkali metals. UO2 powders were dissolved in solutions containing XOH (X = Li, Na, K) and 30% H2O2. Inductively coupled plasma optical emission spectrometry (ICP-OES) measurements of solutions revealed linear trends of uranium versus alkali concentration in solutions resulting from oxidative dissolution of UO2, with X:U molar ratios of 1.0, showing that alkali availability determines the U concentrations in solution. The maximum U concentration in solution was 4.20 × 105 parts per million (ppm), which is comparable to concentrations attained by dissolving UO2 in boiling nitric acid, and was achieved by lithium hydroxide promoted dissolution. Raman spectroscopy and electrospray ionization mass spectrometry (ESI-MS) of solutions indicate that dissolution is accompanied by the formation of various uranyl peroxide cluster species, the identity of which is alkali concentration dependent, revealing remarkably complex speciation at high concentrations of base.

Computationally-Guided Assignment of Unexpected Signals in the Raman Spectra of Uranyl Triperoxide Complexes.

Combination of uranium, peroxide, and mono- (Na, K) or divalent (Mg, Ca, Sr) cations under alkaline aqueous conditions results in the rapid formation of anionic uranyl triperoxide monomers (UTs), (UO2(O2)3)4-, exhibiting unique Raman signatures. Electronic structure calculations were decisive for the interpretation of the spectra and assignment of unexpected signals associated with vibrations of the uranyl and peroxide ions. Assignments were verified by 18O isotopic labeling of the uranyl ions supporting the computational-based interpretation of the experimentally observed peaks and the assignment of a novel asymmetric vibration of the peroxide ligands, v2(O22-).

Single-Crystal Time-of-Flight Neutron Diffraction and Magic-Angle-Spinning NMR Spectroscopy Resolve the Structure and 1H and 7Li Dynamics of the Uranyl Peroxide Nanocluster U60.

Single-crystal time-of-flight neutron diffraction has provided atomic resolution of H atoms of H2O molecules and hydroxyl groups, as well as Li cations in the uranyl peroxide nanocluster U60. Solid-state magic-angle-spinning nuclear magnetic resonance (MAS NMR) spectroscopy was used to confirm the dynamics of these constituents, revealing the transportation of Li atoms and H2O through cluster walls. H atoms of hydroxyl units that are located on the cluster surface are involved in the transfer of H2O and Li cations from inside to outside and vice versa. This exchange occurs as a concerted motion and happens rapidly even in the solid state. As a consequence of its large size and open hexagonal pores, U60 exchanges Li cations more rapidly compared to other uranyl nanoclusters.

Hierarchy of Pyrophosphate-Functionalized Uranyl Peroxide Nanocluster Synthesis.

Herein, we report a new salt of a pyrophosphate-functionalized uranyl peroxide nanocluster {U24Pp12} (1) exhibiting Oh molecular symmetry both in the solid and solution. Study of the system yielding 1 across a wide range of pH by single-crystal X-ray diffraction, small-angle X-ray scattering, and a combination of traditional 31P and diffusion-ordered spectroscopy (DOSY) NMR affords unprecedented insight into the amphoteric chemistry of this uranyl peroxide system. Key results include formation of a rare binary {U24}·{U24Pp12} (3) system observed under alkaline conditions, and evidence of acid-promoted decomposition of {U24Pp12} (1) followed by spatial rearrangement and condensation of {U4} building blocks into the {U32Pp16} (2) cluster. Furthermore, 31P DOSY NMR measurements performed on saturated solutions containing crystalline {U32Pp16} show only trace amounts (∼2% relative abundance) of the intact form of this cluster, suggesting a complex interconversion of {U24Pp12}, {U32Pp16}, and {U4Pp4-x} ions.

Uranyl Peroxide Cage Cluster Solubility in Water and the Role of the Electrical Double Layer.

Uranium concentrations as high as 2.94 × 105 parts per million (1.82 mol of U/1 kg of H2O) occur in water containing nanoscale uranyl cage clusters. The anionic cage clusters, with diameters of 1.5-2.5 nm, are charge-balanced by encapsulated cations, as well as cations within their electrical double layer in solution. The concentration of uranium in these systems is impacted by the countercations (K, Li, Na), and molecular dynamics simulations have predicted their distributions in selected cases. Formation of uranyl cages prevents hydrolysis reactions that would result in formation of insoluble uranyl solids under alkaline conditions, and these spherical clusters reach concentrations that require close packing in solution.

Solution (31)P NMR Study of the Acid-Catalyzed Formation of a Highly Charged {U24Pp12} Nanocluster, [(UO2)24(O2)24(P2O7)12](48-), and Its Structural Characterization in the Solid State Using Single-Crystal Neutron Diffraction.

The first neutron diffraction study of a single crystal containing uranyl peroxide nanoclusters is reported for pyrophosphate-functionalized Na44K6[(UO2)24(O2)24(P2O7)12][IO3]2·140H2O (1). Relative to earlier X-ray studies, neutron diffraction provides superior information concerning the positions of H atoms and lighter counterions. Hydrogen positions have been assigned and reveal an extensive network of H-bonds; notably, most O atoms present in the anionic cluster accept H-bonds from surrounding H2O molecules, and none of the surface-bound O atoms are protonated. The D4h symmetry of the cage is consistent with the presence of six encapsulated K cations, which appear to stabilize the lower symmetry variant of this cluster. (31)P NMR measurements demonstrate retention of this symmetry in solution, while in situ (31)P NMR studies suggest an acid-catalyzed mechanism for the assembly of 1 across a wide range of pH values.

Structure and Reactivity of X-ray Amorphous Uranyl Peroxide, U2O7.

Recent accidents resulting in worker injury and radioactive contamination occurred due to pressurization of uranium yellowcake drums produced in the western U.S.A. The drums contained an X-ray amorphous reactive form of uranium oxide that may have contributed to the pressurization. Heating hydrated uranyl peroxides produced during in situ mining can produce an amorphous compound, as shown by X-ray powder diffraction of material from impacted drums. Subsequently, studtite, [(UO2)(O2)(H2O)2](H2O)2, was heated in the laboratory. Its thermal decomposition produced a hygroscopic anhydrous uranyl peroxide that reacts with water to release O2 gas and form metaschoepite, a uranyl-oxide hydrate. Quantum chemical calculations indicate that the most stable U2O7 conformer consists of two bent (UO2)(2+) uranyl ions bridged by a peroxide group bidentate and parallel to each uranyl ion, and a µ2-O atom, resulting in charge neutrality. A pair distribution function from neutron total scattering supports this structural model, as do (1)H- and (17)O-nuclear magnetic resonance spectra. The reactivity of U2O7 in water and with water in air is higher than that of other uranium oxides, and this can be both hazardous and potentially advantageous in the nuclear fuel cycle.

From aqueous speciation to supramolecular assembly in alkaline earth-uranyl polyoxometalates.

The interplay between aqueous alkaline earth (Ca, Sr, Ba) polycationic speciation and uranyl-peroxide polyoxometalate self-assembly and evolution is described here using solution (Raman and X-ray scattering) and solid-state (microscopy, X-ray diffraction) characterization. Supramolecular assembly of Sr-encapsulated and decorated polyanions and polycations yields the fourth largest inorganic unit cell reported from single-crystal X-ray diffraction.