Valence Fluctuation of Uranium Ions in Uranium Sesquinitride Revealed by Dynamical Mean-field Theory Merged with Density Functional Theory

The electronic properties, in particular, the occupation number of 5f electrons and the valence state of U ions in uranium sesquinitride (U2 N3 ) are studied by using density functional theory (DFT) calculations merged with dynamical mean-field theory (DMFT). The results demonstrate that j=5/2 and j=7/2 manifolds are in the weakly correlated metallic and weakly correlated insulating regimes, respectively. The quasi-particle weights indicate that LS coupling scheme is more feasible for 5f electrons, which are not in the orbital-selective localized state. The weighted summation of the occupation probabilities of 5fn (n=0,1,2,3,4) atomic configurations suggests that 5f electrons have the inter-configuration fluctuation, or the mixed-valence state for U ions, together with an average occupation number of 5f electrons n5f ∼2.234, which is in good agreement with the electron localization function (ELF) and occupation analysis based on other DFT-based calculations. The 5fn -mixing-driven inter-configuration fluctuation might originate from the dual nature of 5f electrons, and the flexible electronic configuration of U ions. Finally, the so-called quasiparticle band structure is also discussed.

Location of Artinite (Mg2CO3(OH)2·3H2O) within the MgO-CO2-H2O system using ab initio thermodynamics

The MgO-CO₂-H₂O system have a variety of important industrial applications including in catalysis, immobilisation of radionuclides and heavy metals, construction, and mineralisation and permanent storage of anthropogenic CO₂. Here, we develop a computational approach to generate phase stability plots for the MgO-CO₂-H₂O system that do not rely on traditional experimental corrections for the solid phases. We compare the predictions made by several dispersion-corrected density-functional theory schemes, and we include the temperature-dependent Gibbs free energy through the quasi-harmonic approximation. We locate the Artinite phase (Mg₂CO₃(OH)₂·3H₂O) within the MgO-CO₂-H₂O phase stability plot, and we demonstrate that this widely-overlooked hydrated and carbonated phase is metastable and can be stabilised by inhibiting the formation of fully-carbonated stable phases. Similar considerations may apply more broadly to other lesser known phases. These findings provide new insight to explain conflicting results from experimental studies, and demonstrate how this phase can potentially be stabilised by optimising the synthesis conditions.

State-of-the-art progress for the selective crystallization of actinides, synthesis of actinide compounds and their functionalization

Crystallization and immobilization of actinides to form actinide compounds are of significant importance for the extraction and reutilization of nuclear waste in the nuclear industry. In this paper, the state-of-art progress in the crystallization of actinides are summarized, as well as the main functionalization of the actinide compounds, i.e., as adsorbents for heavy metal ions and organic pollutant in waste management, as (photo)catalysts for organic degradation and conversion, including degradation of organic dyes and antibiotics, dehydrogenation of N-heterocycles, CO₂ cycloaddition, selective alcohol oxidation and selective oxidation of sulfides. This review will give a comprehensive summary about the synthesis and application exploration of solid actinide crystalline salts and actinide-based metal organic frameworks in the past decades. Finally, the future perspectives and challenges are proposed in the end to give a promising direction for future investigation.

Charge Localization and Magnetic Correlations in the Refined Structure of U3O7

Atomic arrangements in the mixed-valence oxide U₃O₇ are refined from high-resolution neutron scattering data. The crystallographic model describes a long-range structural order in a U₆₀O₁₄₀ primitive cell (space group P4₂/n) containing distorted cuboctahedral oxygen clusters. By combining experimental data and electronic structure calculations accounting for spin-orbit interactions, we provide robust evidence of an interplay between charge localization and the magnetic moments carried by the uranium atoms. The calculations predict U₃O₇ to be a semiconducting solid with a band gap of close to 0.32 eV, and a more pronounced charge-transfer insulator behavior as compared to the well-known Mott insulator UO₂. Most uranium ions (56 out of 60) occur in 9-fold and 10-fold coordinated environments, surrounding the oxygen clusters, and have a tetravalent (24 out of 60) or pentavalent (32 out of 60) state. The remaining uranium ions (4 out of 60) are not contiguous to the oxygen cuboctahedra and have a very compact, 8-fold coordinated environment with two short (2 × 1.93(3) Å) "oxo-type" bonds. The higher Hirshfeld charge and the diamagnetic character point to a hexavalent state for these four uranium ions. Hence, the valence state distribution corresponds to 24/60 × U(IV) + 32/60 U(V) + 4/60 U(VI). The tetravalent and pentavalent uranium ions are predicted to carry noncollinear magnetic moments (with amplitudes of 1.6 and 0.8 μ_B, respectively), resulting in canted ferromagnetic order in characteristic layers within the overall fluorite-related structure.

Synthesis, Characterization and Visible-Light Photocatalytic Activity of Solid and TiO2-Supported Uranium Oxycompounds

In this study, various solid uranium oxycompounds and TiO₂-supported materials based on nanocrystalline anatase TiO₂ are synthesized using uranyl nitrate hexahydrate as a precursor. All uranium-contained samples are characterized using N₂ adsorption, XRD, UV–vis, Raman, TEM, XPS and tested in the oxidation of a volatile organic compound under visible light of the blue region to find correlations between their physicochemical characteristics and photocatalytic activity. Both uranium oxycompounds and TiO₂-supported materials are photocatalytically active and are able to completely oxidize gaseous organic compounds under visible light. If compared to the commercial visible-light TiO₂ KRONOS^® vlp 7000 photocatalyst used as a benchmark, solid uranium oxycompounds exhibit lower or comparable photocatalytic activity under blue light. At the same time, uranium compounds contained uranyl ion with a uranium charge state of 6+, exhibiting much higher activity than other compounds with a lower charge state of uranium. Immobilization of uranyl ions on the surface of nanocrystalline anatase TiO₂ allows for substantial increase in visible-light activity. The photonic efficiency of reaction over uranyl-grafted TiO₂, 12.2%, is 17 times higher than the efficiency for commercial vlp 7000 photocatalyst. Uranyl-grafted TiO₂ has the potential as a visible-light photocatalyst for special areas of application where there is no strict control for use of uranium compounds (e.g., in spaceships or submarines).

Structural and Optical Properties of Phase-Pure UO2, α-U3O8, and α-UO3 Epitaxial Thin Films Grown by Pulsed Laser Deposition

Fundamental understanding of the electronic, chemical, and structural properties of uranium oxides requires the synthesis of high-crystalline-quality epitaxial films of different polymorphs of one material or different phases with various oxygen valence states. We report the growth of single-phase epitaxial UO₂, α-U₃O₈, and α-UO₃ thin films using pulsed laser deposition. Both oxygen partial pressure and substrate temperature play critical roles in determining the crystal structure of the uranium oxide films. X-ray diffraction and Raman spectroscopy demonstrate that the films are single phase with excellent crystallinity and epitaxially grown on a variety of substrates. Chemical valance states and optical properties of epitaxial uranium oxide films are studied by X-ray photoelectron spectroscopy and UV-vis spectroscopy, which further confirm the high-quality stoichiometric phase-pure uranium oxide thin films. Epitaxial UO₂ films show a direct band gap of 2.61 eV, while epitaxial α-U₃O₈ and α-UO₃ films exhibit indirect band gaps of 1.89 and 2.26 eV, respectively. The ability to grow high-quality epitaxy actinide oxide thin films and to access their different phases and polymorphous will have significant benefits to the future applications in nuclear science and technology.

Defect segregation facilitates oxygen transport at fluorite UO2 grain boundaries

An important challenge for modelling transport in materials for energy applications is that in most applications they are polycrystalline, and hence it is critical to understand the properties in the presence of grain boundaries. Moreover, most grain boundaries are not pristine stoichiometric interfaces and hence dopants are likely to play a significant role. In this paper, we describe our recent work on using atomistic molecular dynamics simulations to model the effect of doped grain boundaries on oxygen transport of fluorite structured UO₂. UO₂, much like other fluorite grain boundaries, are found to be sinks for oxygen vacancy segregation relative to the grain interior, thus facilitating oxygen transport. Fission products further enhance diffusivity via strong interactions between the impurities and oxygen defects. Doping produces a striking structural alteration in the Σ5 class of grain boundaries that enhances oxygen diffusivity even further. This article is part of a discussion meeting issue 'Energy materials for a low carbon future'.

Impact of Hydrogen on the Intermediate Oxygen Clusters and Diffusion in Fluorite Structured UO2+ x

Uranium dioxide is the most prevalent nuclear fuel. Defect clusters are known to be present in significant concentrations in hyperstoichoimetric uranium oxide, UO_{2+ x}, and have a significant impact on the corrosion of the material. A detailed understanding of the defect clusters that form is required for accurate diffusion models in UO_{2+ x}. Using ab initio calculations, we show that at low excess oxygen concentration, where defects are mostly isolated oxygen interstitials, hydrogen stabilizes the initial clustering. The simplest cluster at this low excess oxygen stoichiometry consists of a pair of oxygen ions bound to an oxygen vacancy, namely the split mono-interstital, which resembles larger split interstitials clusters in UO_{2+ x}. Our data shows that, depending on local hydrogen concertation, the presence of hydrogen stabilizes this cluster over isolated oxygen interstitials.

Theoretical prediction of some layered Pa2O5 phases: structure and properties

Density functional theory (DFT) was used to predict and study protactinium pentoxide (Pa₂O₅), which presents a fluorite and layered protactinium oxide-type structure. Although the layered structure has been observed with the isostructural transition Nb and Ta metal pentoxides experimentally, the detailed structure and properties of the layered Pa₂O₅ are not clear and understandable. Our theoretical prediction explored some possible stable structures of the Pa₂O₅ stoichiometry according to the existing M₂O₅ structures (where M is an actinide Np or transition Nb, Ta, and V metal) and replacing the M ions with protactinium ions. The structural, mechanical, thermodynamic and electronic properties including lattice parameters, bulk moduli, elastic constants, entropy and band gaps were predicted for all the simulated structures. Pa₂O₅ in the β-V₂O₅ structure was found to be a competitive structure in terms of stability, whereas Pa₂O₅ in the ζ-Nb₂O₅ structure was found to be the most stable overall. This is consistent with Sellers's experimental observations. In particular, Pa₂O₅ in the ζ-Nb₂O₅ structure is predicted to be charge-transfer insulators. Furthermore, we predict that ζ-Nb₂O₅-structured Pa₂O₅ is the most thermodynamically stable under ambient conditions and pressure.

Density functional theory (DFT) was used to predict and study protactinium pentoxide (Pa₂O₅), which presents a fluorite and layered protactinium oxide-type structure.

Direct observation of pure pentavalent uranium in U2O5 thin films by high resolution photoemission spectroscopy

Thin films of the elusive intermediate uranium oxide U₂O₅ have been prepared by exposing UO₃ precursor multilayers to atomic hydrogen. Electron photoemission spectra measured about the uranium 4f core-level doublet contain sharp satellites separated by 7.9(1) eV from the 4f main lines, whilst satellites characteristics of the U(IV) and U(VI) oxidation states, expected respectively at 6.9(1) and 9.7(1) eV from the main 4f lines, are absent. This shows that uranium ions in the films are in a pure pentavalent oxidation state, in contrast to previous investigations of binary oxides claiming that U(V) occurs only as a metastable intermediate state coexisting with U(IV) and U(VI) species. The ratio between the 5f valence band and 4f core-level uranium photoemission intensities decreases by about 50% from UO₂ to U₂O₅, which is consistent with the 5f ² (UO₂) and 5f ¹ (U₂O₅) electronic configurations of the initial state. Our studies conclusively establish the stability of uranium pentoxide.

Structural Changes in the Local Environment of Uranium Atoms in the Three Phases of U4O9

The crystal structure of U4O9 remains an enigma because of its differences with U(4+) and U(5+) coordination polyhedral mixtures, as shown in the XANES experimental results. To better understand this crystal structure, its diffraction pattern was measured at seven different temperatures using neutron diffraction before being independently refined by Rietveld's method and pair distribution function analysis. The O cuboctahedron-a structural element consisting of 13 oxygen atoms-is a specific feature of the U4O9 crystal structure. The volume of the cuboctahedron decreases when the temperature increases, whereas the overall volume of the crystal cell increases. This feature can be correlated with the two U4O9 phase transitions that induce sharp changes in the cuboctahedron geometry, suggesting that this structural element has internal dynamics. In particular, these structural modifications in the γ phase suggest that the high-temperature phase can be described as a mixture of U(4+) and U(5+) coordination polyhedra, the latter having U-O distances shorter than 2.2 Å, that are absent in the former. These changes in uranium polyhedra as a function of temperature are tentatively interpreted using steric arguments. They also raise the question of charge localization on the different U ion sites in the low-temperature phases of U4O9.

Hydride ion formation in stoichiometric UO2

We investigated hydrogen solubility in UO₂ using DFT and predicted that hydrogen species energetically prefers to exist as a hydride ion rather than a proton in a hydroxyl group and on diffusion hydrogen's charge state will change.