The electrocatalytic activity for the hydrogen evolution reaction on alloys is determined by element-specific adsorption sites rather than d-band properties.

Trends of the electrocatalytic activities for the hydrogen evolution reaction (HER) across transition metals are typically explained by d-band properties such as center or upper edge positions in relation to Fermi levels. Here, the universality of this relation is questioned for alloys, exemplified for the AuPt system which is examined with electrocatalytic measurements and density functional theory (DFT) calculations. At small overpotentials, linear combinations of the pure-metals' Tafel kinetics normalized to the alloy compositions are found to precisely resemble the measured HER activities. DFT calculations show almost neighbor-independent adsorption energies on Au and Pt surface-sites, respectively, as the adsorbed hydrogen influences the electron density mostly locally at the adsorption site itself. In contrast, the density of states of the d-band describe the delocalized conduction electrons in the alloys, which are unable to portray the local electronic environments at adsorption sites and related bonding strengths. The adsorption energies at element-specific surface sites are related to overpotential-dependent reaction mechanisms in a multidimensional reinterpretation of the volcano plot for alloys, which bridges the found inconsistencies between activity and bonding strength descriptors of the common electrocatalytic theory for alloys.

A radiochemical lab-on-a-chip paired with computer vision to unlock the crystallization kinetics of (Ba,Ra)SO4.

(Ra,Ba)SO4 solid solutions are commonly encountered as problematic scales in subsurface energy-related applications, e.g., geothermal systems, hydraulic fracturing, conventional oil and gas, etc. Despite its relevance, its crystallization kinetics were never determined because of radium (226), high radioactivity (3.7 × 1010 Bq g-1), and utilization in contemporary research, therefore constrained to trace amounts (< 10-8 M) with the composition of BaxRa1-xSO4 commonly restricted to x > 0.99. What if lab-on-a-chip technology could create new opportunities, enabling the study of highly radioactive radium beyond traces to access new information? In this work, we developed a lab-on-a-chip experiment paired with computer vision to evaluate the crystal growth rate of (Ba,Ra)SO4 solid solutions. The computer vision algorithm enhances experimental throughput, yielding robust statistical insights and further advancing the efficiency of such experiments. The 3D analysis results of the precipitated crystals using confocal Raman spectroscopy suggested that {210} faces grew twice as fast as {001} faces, mirroring a common observation reported for pure barite. The crystal growth rate of (Ba0.5Ra0.5)SO4 follows a second-order reaction with a kinetic constant equal to (1.23 ± 0.09) × 10-10 mol m-2 s-1.

Entropy-Mediated Stable Structural Evolution of Prussian White Cathodes for Long-Life Na-Ion Batteries.

The high-entropy approach is applied to monoclinic Prussian White (PW) Na-ion cathodes to address the issue of unfavorable multilevel phase transitions upon electrochemical cycling, leading to poor stability and capacity decay. A series of Mn-based samples with up to six metal species sharing the N-coordinated positions was synthesized. The material of composition Na1.65 Mn0.4 Fe0.12 Ni0.12 Cu0.12 Co0.12 Cd0.12 [Fe(CN)6 ]0.92 □0.08 ⋅ 1.09H2 O was found to exhibit superior cyclability over medium/low-entropy and conventional single-metal PWs. We also report, to our knowledge for the first time, that a high-symmetry crystal structure may be advantageous for high-entropy PWs during battery operation. Computational comparisons of the formation enthalpy demonstrate that the compositionally less complex materials are prone to phase transitions, which negatively affect cycling performance. Based on data from complementary characterization techniques, an intrinsic mechanism for the stability improvement of the disordered PW structure upon Na+ insertion/extraction is proposed, namely the dual effect of suppression of phase transitions and mitigation of gas evolution.

Low-spin state of Fe in Fe-doped NiOOH electrocatalysts.

Doping with Fe boosts the electrocatalytic performance of NiOOH for the oxygen evolution reaction (OER). To understand this effect, we have employed state-of-the-art electronic structure calculations and thermodynamic modeling. Our study reveals that at low concentrations Fe exists in a low-spin state. Only this spin state explains the large solubility limit of Fe and similarity of Fe-O and Ni-O bond lengths measured in the Fe-doped NiOOH phase. The low-spin state renders the surface Fe sites highly active for the OER. The low-to-high spin transition at the Fe concentration of ~ 25% is consistent with the experimentally determined solubility limit of Fe in NiOOH. The thermodynamic overpotentials computed for doped and pure materials, η = 0.42 V and 0.77 V, agree well with the measured values. Our results indicate a key role of the low-spin state of Fe for the OER activity of Fe-doped NiOOH electrocatalysts.

Understanding the role of flux, pressure and temperature on polymorphism in ThB2O5.

A novel polymorph of ThB2O5, denoted as ß-ThB2O5, was synthesised under high-temperature high-pressure (HT/HP) conditions. Via single crystal X-ray diffraction measurements, ß-ThB2O5 was found to form a three-dimensional (3D) framework structure where thorium atoms are ten-fold oxygen coordinated forming tetra-capped trigonal prisms. The only other known polymorph of ThB2O5, denoted α, synthesised herein using a known borax, B2O3-Na2B4O7, high temperature solid method, was found to transform to the ß polymorph when exposed to conditions of 4 GPa and ∼900 °C. Compared to the α polymorph, ß-ThB2O5 has smaller molar volume by approximately 12%. Exposing a mixture of the α and ß polymorphs to HT/HP conditions ex situ further demonstrated the preferred higher-pressure phase being ß, with no α phase material being observed via Rietveld refinements against laboratory X-ray powder diffraction (PXRD) measurements. In situ heating PXRD measurements on α-ThB2O5 from RT to 1030 °C indicated that α-ThB2O5 transforms to the ß variant at approximately 900 °C via a 1st order mechanism. ß-ThB2O5 was found to exist only over a narrow temperature range, decomposing above 1050 °C. Ab initio calculations using density functional theory (DFT) with the Hubbard U parameter indicated, consistent with experimental observations, that ß is both the preferred phase at higher temperatures and high pressures. Interestingly, it was found by switching from B2O3-Na2B4O7 to H3BO3-Li2CO3 flux using consistent high temperature solid state conditions for the synthesis of the α variant, ß-ThB2O5 could be generated. Comparison of their single crystal measurements showed this was identical to that obtained from HT/HP conditions.

Monazite-Type SmPO4 as Potential Nuclear Waste Form: Insights into Radiation Effects from Ion-Beam Irradiation and Atomistic Simulations.

Single-phase monazite-type ceramics are considered as potential host matrices for the conditioning of separated plutonium and minor actinides. Sm-orthophosphates were synthesised and their behaviour under irradiation was investigated with respect to their long-term performance in the repository environment. Sintered SmPO4 pellets and thin lamellae were irradiated with 1, 3.5, and 7 MeV Au ions, up to fluences of 5.1 × 1014 ions cm-2 to simulate ballistic effects of recoiling nuclei resulting from α-decay of incorporated actinides. Threshold displacement energies for monazite-type SmPO4 subsequently used in SRIM/TRIM simulations were derived from atomistic simulations. Raman spectra obtained from irradiated lamellae revealed vast amorphisation at the highest fluence used, although local annealing effects were observed. The broadened, but still discernible, band of the symmetrical stretching vibration in SmPO4 and the negligible increase in P-O bond lengths suggest that amorphisation of monazite is mainly due to a breaking of Ln-O bonds. PO4 groups show structural disorder in the local environment but seem to behave as tight units. Annealing effects observed during the irradiation experiment and the distinctively lower dose rates incurred in actinide bearing waste forms and potential α-radiation-induced annealing effects indicate that SmPO4-based waste forms have a high potential for withstanding amorphisation.

Gamma radiolytic stability of the novel modified diglycolamide 2,2'-oxybis(N,N-didecylpropanamide) (mTDDGA) for grouped actinide extraction.

Reprocessing of spent nuclear fuel aims at improving resource efficiency and reducing its radiotoxicity and heat production in the long term. The necessary separation of certain metal ions from the spent fuel solutions can be achieved using different solvent extraction processes. For the scenario of the EURO-GANEX process, the use of the new, modified diglycolamide 2,2'-oxybis(N,N-didecylpropanamide) (mTDDGA) was recently proposed to simplify the current solvent composition and reduce extraction of fission products. Before further developing the process based on this new ligand, its stability under ionizing radiation conditions needs to be studied. For this reason, gamma irradiation experiments were conducted followed by analyses with high performance liquid chromatography coupled to a mass spectrometer (HPLC-MS). The determined degradation rate of mTDDGA was found to be lower than that of the reference molecule N,N,N',N'-tetra-n-octyl-diglycolamide (TODGA). Many identified degradation compounds of both molecules are analogues showing the same bond breaking, although also unreported de-methylation, double/triple de-alkylation and n-dodecane addition products were observed.

The Structure of the Electric Double Layer of the Protic Ionic Liquid [Dema][TfO] Analyzed by Atomic Force Spectroscopy.

Protic ionic liquids are promising electrolytes for fuel cell applications. They would allow for an increase in operation temperatures to more than 100 °C, facilitating water and heat management and, thus, increasing overall efficiency. As ionic liquids consist of bulky charged molecules, the structure of the electric double layer significantly differs from that of aqueous electrolytes. In order to elucidate the nanoscale structure of the electrolyte-electrode interface, we employ atomic force spectroscopy, in conjunction with theoretical modeling using molecular dynamics. Investigations of the low-acidic protic ionic liquid diethylmethylammonium triflate, in contact with a platinum (100) single crystal, reveal a layered structure consisting of alternating anion and cation layers at the interface, as already described for aprotic ionic liquids. The structured double layer depends on the applied electrode potential and extends several nanometers into the liquid, whereby the stiffness decreases with increasing distance from the interface. The presence of water distorts the layering, which, in turn, significantly changes the system's electrochemical performance. Our results indicate that for low-acidic ionic liquids, a careful adjustment of the water content is needed in order to enhance the proton transport to and from the catalytic electrode.

Pyrochlore Compounds From Atomistic Simulations.

Pyrochlore compounds (A 2 B 2O7) have a large applicability in various branches of science and technology. These materials are considered for use as effective ionic conductors for solid state batteries or as matrices for immobilization of actinide elements, amongst many other applications. In this contribution we discuss the simulation-based effort made in the Institute of Energy and Climate Research at Forschungszentrum Jülich and partner institutions regarding reliable computation of properties of pyrochlore and defect fluorite compounds. In the scope of this contribution, we focus on the investigation of dopant incorporation, defect formation and anion migration, as well as understanding of order-disorder transitions in these compounds. We present new, accurate simulated data on incorporation of U, Np, Pu, Am and Cm actinide elements into pyrochlores, activation energies for oxygen migration and radiation damage-induced structural changes in these materials. All the discussed simulation results are combined with available experimental data to provide a reliable description of properties of investigated materials. We demonstrate that a synergy of computed and experimental data leads to a superior characterization of pyrochlores, which could not be easily achieved by either of these methods when applied separately.

Properties of the Pt(111)/electrolyte electrochemical interface studied with a hybrid DFT-solvation approach.

Self-consistent modeling of the interface between solid metal electrode and liquid electrolyte is a crucial challenge in computational electrochemistry. In this contribution, we adopt the effective screening medium reference interaction site method (ESM-RISM) to study the charged interface between a Pt(111) surface that is partially covered with chemisorbed oxygen and an aqueous acidic electrolyte. This method proves to be well suited to describe the chemisorption and charging state of the interface at controlled electrode potential. We present an in-depth assessment of the ESM-RISM parameterization and of the importance of computing near-surface water molecules explicitly at the quantum mechanical level. We found that ESM-RISM is able to reproduce some key interface properties, including the peculiar, non-monotonic charging relation of the Pt(111)/electrolyte interface. The comparison with independent theoretical models and explicit simulations of the interface reveals strengths and limitations of ESM-RISM for modeling electrochemical interfaces.

Achieving and Stabilizing Uranyl Bending via Physical Pressure.

Applying physical pressure in the uranyl-sulfate system has resulted in the formation of the first purely inorganic uranyl oxo-salt phase with a considerable uranyl bend: Na4[(UO2)(SO4)3]. In addition to a strong bend of the typically almost linear O═U═O, the typically equatorial plane is broken up by two out-of-plane oxygen positions. Computational investigations show the origin of the bending to lie in the applied physical pressure and not in the electronic influence or steric hindrance. The increase in pressure onto the system has been shown to increase uranyl bending. Furthermore, the phase formation is compared with a reference phase of a similar structure without uranyl bending, and a transition pressure of 2.5 GPa is predicted, which is well in agreement with the experimental results.

Tilting and Distortion in Rutile-Related Mixed Metal Ternary Uranium Oxides: A Structural, Spectroscopic, and Theoretical Investigation.

A systematic investigation examining the origins of structural distortions in rutile-related ternary uranium AUO4 oxides using a combination of high-resolution structural and spectroscopic measurements supported by ab initio calculations is presented. The structures of ß-CdUO4, MnUO4, CoUO4, and MgUO4 are determined at high precision by using a combination of neutron powder diffraction (NPD) and synchrotron X-ray powder diffraction (S-XRD) or single crystal X-ray diffraction. The structure of ß-CdUO4 is best described by space group Cmmm whereas MnUO4, CoUO4, and MgUO4 are described by the lower symmetry Ibmm space group and are isostructural with the previously reported ß-NiUO4 [Murphy et al. Inorg. Chem. 2018, 57, 13847]. X-ray absorption spectroscopy (XAS) analysis shows all five oxides contain hexavalent uranium. The difference in space group can be understood on the basis of size mismatch between the A2+ and U6+ cations whereby unsatisfactory matching results in structural distortions manifested through tilting of the AO6 polyhedra, leading to a change in symmetry from Cmmm to Ibmm. Such tilts are absent in the Cmmm structure. Heating the Ibmm AUO4 oxides results in reduction of the tilt angle. This is demonstrated for MnUO4 where in situ S-XRD measurements reveal a second-order phase transition to Cmmm near T = 200 °C. Based on the extrapolation of variable temperature in situ S-XRD data, CoUO4 is predicted to undergo a continuous phase transition to Cmmm at ∼1475 °C. Comparison of the measured and computed data highlights inadequacies in the DFT+U approach, and the conducted analysis should guide future improvements in computational methods. The results of this investigation are discussed in the context of the wider AUO4 family of oxides.

Americium incorporation into studtite: a theoretical and experimental study.

Studtite, [UO2(η2-O2)(H2O)2]·2H2O, and metastudtite, [UO2(η2-O2)(H2O)2], are important phase alterations of UO2 in a spent nuclear fuel repository and have previously been shown to react with Np(v). In this work we extend the study to Am(v) on a tracer scale and show spectroscopic evidence that the Am is incorporated into the structure of studtite as Am(iii). A computational study on the possible mechanisms for the incorporation of Np and Am shows that protonation of the -yl oxygen is the favoured route and the calculated incorporation energies are large and positive. The results suggest that Am is less favoured compared to Np but energetically more favoured to incorporate both actinide ions into metastudtite rather than studtite. Finally, we have shown that once incorporated, Am readily leaches into water but spectroscopic measurements suggest subtle changes in the structure of studtite.

Controlling Oxygen Defect Formation and Its Effect on Reversible Symmetry Lowering and Disorder-to-Order Phase Transformations in Nonstoichiometric Ternary Uranium Oxides.

In situ synchrotron powder X-ray diffraction measurements have demonstrated that the isostructural AUO4- x ( A = alkaline earth metal cation) oxides CaUO4- x and α-Sr0.4Ca0.6UO4- x undergo a reversible phase transformation under reducing conditions at high temperatures associated with the ordering of in-plane oxygen vacancies resulting in the lowering of symmetry. When rhombohedral (space group R3̅ m) CaUO4- x and α-Sr0.4Ca0.6UO4- x are heated to 450 and 400 °C, respectively, in a hydrogen atmosphere, they undergo a first-order phase transformation to a single phase structure which can be refined against a triclinic model in space group P1̅, δ-CaUO4- x and δ-Sr0.4Ca0.6UO4- x, where the oxygen vacancies are disordered initially. Continued heating results in the appearance of superlattice reflections, indicating the ordering of in-plane oxygen vacancies. Cooling ordered δ-CaUO4- x and δ-Sr0.4Ca0.6UO4- x to near room temperature results in the reformation of the disordered rhombohedral phases. Essential to the transformation is the generation of a critical amount of oxygen vacancies. Once these are formed, the transformation can be accessed continuously through thermal cycling, showing that the transformations are purely thermodynamic in origin. Stoichiometric structures of both oxides can be recovered by heating oxygen deficient CaUO4- x and α-Sr0.4Ca0.6UO4- x under pure oxygen to high temperatures. When heated in air, the amount of oxygen vacancy defects that form in CaUO4- x and α-Sr0.4Ca0.6UO4- x are found to correlate with the A site composition. The inclusion of the larger Sr2+ cation on the A site reduces defect-defect interactions, which increases the amount of defects that can form and lowers their formation temperature. The relative difference in the amount of defects that form can be understood on the basis of oxygen vacancy and U5+ disordering as shown by both ab initio calculations and estimated oxygen vacancy formation energies based on thermodynamic considerations. This difference in defect-defect interactions consequently introduces variations in the long-range ordered anionic lattice of the δ phases despite the isostructural relationship of the α structures of CaUO4- x and Sr0.4Ca0.6UO4- x. These results are discussed with respect to the influence the A site cation has upon anion defect formation and ordering and are also compared to δ-SrUO4- x, the only other material known to be able to undergo a reversible symmetry lowering and disorder-to-order transformation with increasing temperature.

Rare-Earth Orthophosphates From Atomistic Simulations.

Lanthanide phosphates (LnPO 4) are considered as a potential nuclear waste form for immobilization of Pu and minor actinides (Np, Am, and Cm). In that respect, in the recent years we have applied advanced atomistic simulation methods to investigate various properties of these materials on the atomic scale. In particular, we computed several structural, thermochemical, thermodynamic and radiation damage related parameters. From a theoretical point of view, these materials turn out to be excellent systems for testing quantum mechanics-based computational methods for strongly correlated electronic systems. On the other hand, by conducting joint atomistic modeling and experimental research, we have been able to obtain enhanced understanding of the properties of lanthanide phosphates. Here we discuss joint initiatives directed at understanding the thermodynamically driven long-term performance of these materials, including long-term stability of solid solutions with actinides and studies of structural incorporation of f elements into these materials. In particular, we discuss the maximum load of Pu into the lanthanide-phosphate monazites. We also address the importance of our results for applications of lanthanide-phosphates beyond nuclear waste applications, in particular the monazite-xenotime systems in geothermometry. For this we have derived a state-of-the-art model of monazite-xenotime solubilities. Last but not least, we discuss the advantage of usage of atomistic simulations and the modern computational facilities for understanding of behavior of nuclear waste-related materials.

Unprecedented Inversion of Selectivity and Extraordinary Difference in the Complexation of Trivalent f†Elements by Diastereomers of a Methylated Diglycolamide.

When considering f elements, solvent extraction is primarily used for the removal of lanthanides from ore and their recycling, as well as for the separation of actinides from used nuclear fuel. Understanding the complexation mechanism of metal ions with organic extractants, particularly the influence of their molecular structure on complex formation is of fundamental importance. Herein, we report an extraordinary (up to two orders of magnitude) change in the extraction efficiency of f elements with two diastereomers of dimethyl tetraoctyl diglycolamide (Me2 -TODGA), which only differ in the orientation of a single methyl group. Solvent extraction techniques, extended X-ray absorption fine structure (EXAFS) measurements, and density functional theory (DFT) based ab initio calculations were used to understand their complex structures and to explain their complexation mechanism. We show that the huge differences observed in extraction selectivity results from a small change in the complexation of nitrate counter-ions caused by the different orientation of one methyl group in the backbone of the extractant. The obtained results give a significant new insight into metal-ligand complexation mechanisms, which will promote the development of more efficient separation techniques.

Trends in the valence band electronic structures of mixed uranium oxides.

The valence band electronic structures of mixed uranium oxides (UO2, U4O9, U3O7, U3O8, and ß-UO3) have been studied using the resonant inelastic X-ray scattering (RIXS) technique at the U M5 edge and computational methods. We show here that the RIXS technique and recorded U 5f-O 2p charge transfer excitations can be used to test the validity of theoretical approximations.

A Spectroscopic and Computational Study of Cm3+ Incorporation in Lanthanide Phosphate Rhabdophane (LnPO4·0.67H2O) and Monazite (LnPO4).

This study investigates the incorporation of the minor actinide curium (Cm3+) in a series of synthetic La1- xGd xPO4 ( x = 0, 0.24, 0.54, 0.83, 1) monazite and rhabdophane solid-solutions. To obtain information on the incorporation process on the molecular scale and to understand the distribution of the dopant in the synthetic phosphate phases, combined time-resolved laser fluorescence spectroscopy and X-ray absorption fine structure spectroscopy investigations were conducted and complemented with ab initio atomistic simulations. We found that Cm3+ is incorporated in the monazite endmembers (LaPO4 and GdPO4) on one specific, highly ordered lattice site. The intermediate solid-solutions, however, display increasing disorder around the Cm3+ dopant as a result of random variations in nearest neighbor distances. In hydrated rhabdophane, and especially its La-rich solid-solutions, Cm3+ is preferentially incorporated on nonhydrated lattice sites. This site occupancy is not in agreement with the hydrated rhabdophane structure, where two-thirds of the lattice sites are associated with water of hydration (LnPO4·0.67H2O), implying that structural substitution reactions cannot be predicted based on the structure of the host matrix only.

Unexpected Crystallographic Phase Transformation in Nonstoichiometric SrUO4- x: Reversible Oxygen Defect Ordering and Symmetry Lowering with Increasing Temperature.

In situ synchrotron powder X-ray diffraction measurements have demonstrated that SrUO4 undergoes a reversible phase transformation under reducing conditions at high temperatures, associated with the ordering of oxygen defects resulting in a lowering of crystallographic symmetry. When substoichiometric rhombohedral α-SrUO4- x, in space group R3̅ m with disordered in-plane oxygen defects, is heated above 200 °C in a hydrogen atmosphere it undergoes a first order phase transformation to a (disordered) triclinic polymorph, δ-SrUO4- x, in space group P1̅. Continued heating to above 450 °C results in the appearance of superlattice reflections, due to oxygen-vacancy ordering forming an ordered structure δ-SrUO4- x. Cooling δ-SrUO4- x toward room temperature results in the reformation of the rhombohedral phase α-SrUO4- x with disordered defects, confirming the reversibility of the transformation. This suggests that the transformation, resulting from oxygen vacancy ordering, is not a consequence of sample reduction or decomposition, but rather represents a change in the energetics of the system. A strong reducing atmosphere is required to generate a critical amount of oxygen defects in α-SrUO4- x to enable the transformation to δ-SrUO4- x but once formed the transformation between these two phases can be induced by thermal cycling. The structure of δ-SrUO4- x at 1000 °C was determined using symmetry representation analysis, with the additional reflections indexed to a commensurate distortion vector k = ⟨1/4 1/4 3/4⟩. The ordered 2D layered triclinic structure of δ-SrUO4- x can be considered a structural distortion of the disordered 2D layered rhombohedral α-SrUO4- x structure through the preferential rearrangement of the in-plane oxygen vacancies. Ab initio calculations using density functional theory with self-consistently derived Hubbard U parameter support the assigned ordered defect superstructure model. Entropy changes associated with the temperature dependent short-range ordering of the reduced U species are believed to be important and these are discussed with respect to the results of the ab initio calculations.

Dehydration of the Uranyl Peroxide Studtite, [UO2(η2-O2)(H2O)2]·2H2O, Affords a Drastic Change in the Electronic Structure: A Combined X-ray Spectroscopic and Theoretical Analysis.

The minerals studtite, [UO2(η2-O2)(H2O)2]·2H2O, and metastudtite, [UO2(η2-O2)(H2O)2], are uranyl peroxide minerals that are major oxidative alteration phases of UO2 under conditions of geological storage. The dehydration of studtite has been studied using X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy. XPS of the U 4f region shows small but significant differences between studtite and metastudtite, with the 4f binding energy of studtite being the highest reported for a uranyl mineral studied by this technique. Further information about the changes in the electronic structure was elucidated using U M4-edge high-energy resolution X-ray absorption near-edge structure (HR-XANES) spectroscopy, which directly probes f orbital states. The transition from the 3d to 5fσ* orbital is sensitive to variations in the U═Oaxial bond length and to changes in the bond covalency. We report evidence that the covalence in the uranyl fragment decreases upon dehydration. Photoluminescence spectroscopy at near-liquid helium temperatures reveals significant spectral differences between the two materials, correlating with the X-ray spectroscopy results. A theoretical investigation has been conducted on the structures of both studtite and metastudtite and benchmarked to the HR-XANES spectra. These illustrate the sensitivity of the 3d to 5f σ* transition toward U═Oaxial bond variation. Small structural changes upon dehydration have been shown to have an important electronic effect on the uranyl fragment.