Oxidation state is a key chemical quantity that allows the understanding and prediction of the majority of chemical reactions; however, the main deficiency using the formal oxidation state comes from the materials containing multivalent metals. Among them, the most complicated element plutonium (Pu) is an outstanding instance. Here, we calculate the orbital occupation numbers under the frameworks of first-principles density functional theory (DFT) + U methods to quantitatively determine the Pu-oxidation state of the recently reported complex molecular solids: [K(crypt)]Pu[C₅H₃(SiMe₃)₂]₃, Pu₃(DPA)₅(H₂O)₂, and Pu₃(DPA)₆H. The results show that the oxidation state of Pu is an extremely low Pu²⁺ in the former and a mixed Pu³⁺/Pu⁴⁺ in the latter two compounds, which is consistent with the experimental identifications. The steric effects and the environmentally sensitive localization → delocalization transition of Pu 5f electrons can rationally elucidate the formation of the unusual oxidation states. Such atomic-resolution quantitative determination of the unusual Pu oxidation states in the complex molecular solids offers an alternative for the further exploration of peculiar Pu solid-state materials.

Plutonium Oxidation States in Complex Molecular Solids

New theoretical insights into the actual oxidation states of uranium in the solid-state compounds

In order to reveal the correlation effect on the electronic properties in particular 5f electron occupation numbers of Pu/U ions in a (Pu,U) mixed oxide-PuUO4, a first principles calculation is performed by using density functional theory (DFT) plus a dynamical mean field theory (DMFT) scheme with the spin-orbit coupling (SOC) and on-site Coulomb repulsion for correlation effect due to localized Pu/U 5f orbitals. Results demonstrate that Pu/U 5f electron occupation numbers in the ground state of PuUO4 are mainly composed of 5f4/5f5 and 5f2/5f3 configurations, and exhibiting the intermediate occupation (IO) numbers with average 5f occupation numbers of about nf = 4.879 and 2.423 for Pu and U ions, respectively, irrespective of different Pu and U lattice sites in PuUO4. Pu 5f j = 5/2 and j = 7/2 components are in moderately and weakly correlated states, respectively, while U 5f j = 5/2 and j = 7/2 manifolds are both in weakly correlated states. jj and LS coupling schemes are feasible for Pu and U 5f electrons, respectively. In order to directly compare with the experimental angle-resolved photoemission spectrum (ARPES), we also estimate the momentum-resolved electronic spectrum function for this system.

Intermediate occupation numbers for 5f electrons in a Pu and U mixed oxide.

Scaling the existence state of hydrogen in metal binary oxides

An environmentally aged radioactive particle of UFeO4 recovered from soil contaminated with munitions depleted uranium (DU) was characterised by microbeam synchrotron X-ray analysis Imaging of uranium speciation by spatially resolved X-ray diffraction (μ-XRD) and X-ray absorption spectroscopy (μ-XAS) was used to localise UFeO4 in the particle, which was coincident with a distribution of U(V) The U oxidation state was confirmed using X-ray Absorption Near Edge Structure (μ-XANES) spectroscopy as +49 ± 015 Le-Bail fitting of the particle powder XRD pattern confirmed the presence of UFeO4 and a minor alteration product identified as chernikovite (H3O)(UO2)(PO4)·3H2O Refined unit cell parameters for UFeO4 were in good agreement with previously published values Uranium–oxygen interatomic distances in the first co-ordination sphere were determined by fitting of Extended X-ray Absorption Fine Structure (μ-EXAFS) spectroscopy The average first shell U–O distance was 2148 ± 0012 A, corresponding to a U valence of +496 ± 013 using bond valence sum analysis Using bond distances from the published structure of UFeO4, U and Fe bond valence sums were calculated as +500 and +283 respectively, supporting the spectroscopic analysis and confirming the presence of a U(V)/Fe(III) pair Overall this investigation provides important evidence for the stability of U(V) ternary oxides, in oxic, variably moist surface environment conditions for at least 25 years

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Multimodal X-ray microanalysis of a UFeO4: evidence for the environmental stability of ternary U(v) oxides from depleted uranium munitions testing.

Uranium (U) has attracted considerable fundamental and applied studies; among them, the rigorous determination of its chemical state is one of the crucial topics owing to its well-known multivalent...

Atom-Resolved Chemical States in the Multivalent U-TM-O (TM: Ti, V, Cr, Mn, Fe, Ni, Nb, Mo, W) Ternary Oxides from First-Principles

Uranium (U) has attracted considerable fundamental and applied studies; among them, the rigorous determination of its chemical state is one of the crucial topics owing to its well-known multivalent feature associated with complicated chemical bonding. However, the related studies on solid-state U ternary or beyond ternary compounds are quite in lack. Herein, first-principles DFT + U calculations are performed to unambiguously determine the actual oxidation states of metal ions in U ternary oxides containing multivalent transition-metals (TMs: Ti, V, Cr, Mn, Fe, Ni, Nb, Mo, W). For the first time, the actual oxidation states of U and TM in the ternary oxides are obtained in a quantitative and atom-resolved manner, thereby providing their nominal compositions. The results disclose the variations of U oxidation states with TM’s properties, chemical surrounding, local structure, and oxygen vacancy. In terms of charge transfer, electronic structure, orbital energy level, and oxygen vacancy effect, I conclude that the oxidation state is the competitive consequence of those electronic and structural factors and that electron-structure-related factors play more important roles. Because of the increasing orbital expansion from 3d to 4d and 5d TMs, the latter TMs (Nb, Mo, W) have the greater tendency to form high oxidation states and suppress the formation of high oxidation states of U. On the contrary, 3d TMs, especially Cr, Mn, Fe, and Ni, tend to adopt low oxidation states, thereby promoting the emergence of high oxidation states of U.

Bingyun Ao

Papers

Atom-Resolved Chemical States in the Multivalent U-TM-O (TM: Ti, V, Cr, Mn, Fe, Ni, Nb, Mo, W) Ternary Oxides from First-Principles

Atom-Resolved Chemical States in the Multivalent U-TM-O (TM: Ti, V, Cr, Mn, Fe, Ni, Nb, Mo, W) Ternary Oxides from First-Principles