Search

Novel stable H-rich clathrate phases of Np–H system are composed of unusual H cages, and exhibit superconductivity behaviors under high pressure.

The effect of atomic hydrogen adsorption on AnO2 (An = Th, U, and Pu) surfaces is studied in the framework of density functional theory and Hubbard-corrected density functional theory. Several adsorption coverages (1/3, 1/2, 2/3, and 1 monolayer) are considered. For the band insulator ThO2, surface metallicity induced by hydrogen adsorption is observed due to the electron donation of the hydrogen to the surface. But this effect is found to be strongly suppressed by electronic correlation for the Mott insulators UO2 and PuO2 because the electrons from the adsorbed hydrogen atoms occupy the localized 5f orbitals of the surface U/Pu atoms.

The recent breakthrough in the discovery of nearly ferromagnetic spin-triplet superconductivity UTe2 has attracted vivid investigations regarding its physical properties; however, the fundamental-c...

Through matrix isolation infrared spectrometry and quantum chemical calculations, the reactions of laser ablated V, Nb, and Ta with boron trifluoride were investigated in excess solid neon at 4 K. The possible reaction products FBMF2, F2BMF, and BMF3 (M = V, Nb, and Ta) were calculated at the B3LYP, BPW91, and CCSD(T) levels of theory. The B-M bond strength in FBMF2 molecules is confirmed by energy decomposition analysis-natural orbitals for chemical valence calculations, CASSCF calculation, and natural bond orbital analysis, which favors one σ bond and two half π bonds.

The high-pressure processes of plutonium hydrides are usually involved in many practical situations, such as the volume expansion during the hydride formation, the pressure of helium bubbles due to...

The solid-state metal sulfides have versatile technological and industrial applications; however, a comprehensive understanding of their chemical states remains in lack mainly due to the complicated bonding behavior of sulfur. Herein I conduct the systematic first-principles DFT + U calculations on the solid-state metal (all transition metals and the early five actinide metals Th, Pa, U, Np, Pu) binary sulfides, focusing on quantitative determination of the quantum-mechanical oxidation states (OSqm) of metal ions by counting d or f orbital occupation numbers. The results show the variation of OSqm of a specific metal ion with sulfur stoichiometry and the trend of OSqm of all considered metal ions. The most remarkable aspect is that OSqm in many sulfides are not consistent with formal OS (OSf) assigned by ionic approximation, especially for the sulfides with the highest sulfur composition. After detailed analysis of OSqm in actinide sulfides, I conclude that the relatively weak bonding between metal and sulfur, the relatively strong S-S bonding interaction, and the localization/itinerancy dual nature of d and f electrons can rationally elucidate the deviation of OSqm from OSf. Such quantitative determination of OSqm in the binary sulfides is expected to offer an alternative for the further exploration of more complicated sulfides and the theoretical design of novel sulfides.

The crystal structures, phonon spectra, and electronic properties of uranium trioxide (${\mathrm{UO}}_{3}$) under high pressure have been systematically explored using a particle swarm optimization structure prediction method in conjunction with first-principles calculations. Our calculated lattice constants and the transition pressure of the two experimentally reported phases of $\ensuremath{\gamma}$- and $\ensuremath{\eta}\ensuremath{-}{\mathrm{UO}}_{3}$ are consistent with previous experiments. At pressures of 13, 62, 220 GPa, three new structures of $P{6}_{3}$/$mmc$, $Pm\overline{3}n$, and $Fm\overline{3}m$ are predicted in sequence to be thermodynamically stable. Based on our calculated elastic constants and phonon spectra, we indicate that these three phases are mechanically and dynamically stable. Interestingly, upon phase transition from $P{6}_{3}$/$mmc$ to $Pm\overline{3}n$, ${\mathrm{UO}}_{3}$ undergoes a semiconductor-to-metal electronic transition. In addition, we report results of specific heat, entropy, bulk modulus, shear modulus, Young's modulus, Poisson's ratio, and Debye temperature. Our results provide key insights into understanding the structural as well as the electronic behaviors of ${\mathrm{UO}}_{3}$ under the condition of external pressure.

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...

The oxide layer structure over Pu is the central issue of Pu corrosion science; however, the widely discussed sandwich-like structure (Pu//Pu2O3//PuO2) might be just a phenomenological description. Here we perform first-principles calculations with the method verification on Ce oxides, the ideal analogues of Pu oxides, to establish a universal picture of Pu oxide layer structure. The results show that homogeneous PunO2n-2 between PuO2 and Pu2O3 are energetically more favorable than PuO2-x with ordered arrays of oxygen vacancies. Thus, a ladder-like oxide layer structure is proposed to improve a simple sandwich-like model of the Pu oxide layers.

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 atomi...

Fluorite-type XO2 (X = Ce, Th, U) have versatile technological and industrial applications, and the behavior of impurities in the oxides is one of the engaging topics for their application. However, the fundamental behaviors of impurities are still lacking. Herein, we conduct a systematic first-principles DFT+U screening to find the trends of transition metal (TM) behaviors in the three dioxides in terms of energetics and electronic states, with a particular focus on the dependency of f electronic states of the hosts. In order to overcome the long-standing bottleneck of determining the true oxidation state of multivalent TMs, Ce and U, a more rigorous method based on counting orbital occupation numbers of f and d orbitals is performed for clarification. The calculated incorporation energies and formation energies of TMs show that the relative stability of TMs in the three XO2 exhibits similar trends, indicative of the dominant roles played by the host oxides with the same crystal structure and very close lattice parameters. On the other hand, the quantitative differences in the stability and electronic state of doped TMs could be mainly attributed to the differences in the electronic structure of host XO2. The 5f electrons in UO2 are more delocalized than 4f in CeO2, suppressing the formation of high oxidation states of TMs in the former. For ThO2 with the negligible f electrons associated with monovalent Th4+, the doped TMs tend to adopt the oxidation states close to TM4+, achieving the electronically matched states. The appearance of a few unusual oxidation states of TMs sheds light on the flexible delocalization-localization mutual transition of f or d valence electrons.

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.

Ceria and ceria-based materials have versatile technological and industrial applications physically ascribed to the flexible fluctuation of the Ce oxidation state between Ce4+ and Ce3+. Considerabl...

Identifying reaction intermediates in gas-phase investigations will provide understanding for the related catalysts in fundamental aspects including bonding interactions of the reaction species, oxidation states (OSs) of the anchored atoms, and reaction mechanisms. Herein, carbon monoxide (CO) oxidation by praseodymium monoxide (PrO) molecules has been investigated as a model reaction in solid argon using matrix-isolation IR spectroscopy and quantum-chemical calculations. Two reaction intermediates, OPr(η1-CO) and OPr(η2-CO), have been trapped and characterized in argon matrixes. The intermediate OPr(η2-CO) shows an extremely low C-O stretching band at 1624.5 cm-1. Quantum-chemistry studies indicate that the bonding in OPr(η1-CO) is described as "donor-acceptor" interactions conforming to the Dewar-Chatt-Duncanson motif. However, the bonding in OPr(η2-CO) results evidently from a combination of dominant ionic forces and normal Lewis "acid-base" interactions. The electron density of the singly occupied bonding orbital is strongly polarized to the CO fragment in OPr(η2-CO). Electronic structure analysis suggests that the two captured species exhibit Pr(III) OSs. Besides, the pathways of CO oxidation have been discussed.

The effect of atomic hydrogen adsorption on AnO2 (An = Th, U, and Pu) surfaces is studied in the framework of density functional theory and Hubbard-corrected density functional theory. Several adsorption coverages (1/3, 1/2, 2/3, and 1 monolayer) are considered. For the band insulator ThO2, surface metallicity induced by hydrogen adsorption is observed due to the electron donation of the hydrogen to the surface. But this effect is found to be strongly suppressed by electronic correlation for the Mott insulators UO2 and PuO2 because the electrons from the adsorbed hydrogen atoms occupy the localized 5f orbitals of the surface U/Pu atoms.

Through matrix isolation infrared spectrometry and quantum chemical calculations, the reactions of laser ablated V, Nb, and Ta with boron trifluoride were investigated in excess solid neon at 4 K. The possible reaction products FBMF

We investigate the distribution preference of hydrogen in the α-U/UO

Systematic americyl-hydration cations were investigated theoretically to understand the electronic structures and bonding in [(AmO2)(H2O)n]2+/1+ (n = 1–6). We obtained the binding energy using density functional theory methods with scalar relativistic and spin–orbit coupling effects. The geometric structures of these species have been investigated in aqueous solution via an implicit solvation model. Computational results reveal that the complexes of five equatorial water molecules coordinated to americyl ions are the most stable due to the enhanced ionic interactions between the AmO22+/1+ cation and multiple oxygen atoms as electron donors. As expected, Am–Owater bonds in such series are electrostatic in nature and contain a generally decreasing covalent character when hydration number increases.

The existence of condensed plutonium monoxide (PuO) has long been controversial, in which carbon element plays a vital role. In this work, we modeled PuO with carbon substitution of oxygen as the special quasirandom structures of NaCl-type plutonium monoxycarbide (PuOxC1–x) and performed first-principles calculations on these models. It is found that PuO could indeed be stabilized by carbon substitution. The amount of carbon element to stabilize PuO is very small if the source of carbon is CO or CO2. If the carbon is contributed by plutonium monocarbide, the range of stable PuOxC1–x agrees well with the experimental result. By analyzing the electronic structures and chemical bonds, we ascribe the stabilization of PuO by carbon substitution to the newly established Pu 5f/6d–C 2p hybridization around the Fermi level, i.e., stronger Pu–C bonds.

One of the important research contents on hydrogen corrosion of plutonium is the determination of the complex crystal structures of plutonium hydrides and the bonding interactions between plutonium and hydrogen. However, it is very difficult to carry out the structural characterization of plutonium hydrides because of their high activity, high toxicity, and radioactivity. In this work, the crystal structures, lattice vibrations, and bonding properties of plutonium hydrides under ambient pressure are investigated by means of the density functional theory + U approach. Results show that PuH3 exhibits many competition phase structures. After considering spin polarization, strong correlation (U), and spin–orbit coupling effects on the total energy and lattice dynamics stability, it is found that PuH3 at ambient pressure is more likely to be hexagonal P63cm or trigonal P3c1 structure, instead of the usual supposed structures of hexagonal P63/mmc (LaF3-type) and face-centered cubic (BiF3-type). The calculated e...

The activation of ammonia by cerium atom has been investigated in solid argon using infrared spectroscopy and density functional theoretical calculations. The results reveal that the spontaneous formation of CeNH3 complex on annealing is the initial step in the reactions of cerium atoms with ammonia. The CeNH3 complexes rearrange to generate the inserted HCeNH2 molecules on irradiation. A “triplet–singlet” spin conversion occurs along the reaction path in which HCeNH2 (3A″) isomerizes into H2CeNH (1A′). The H2CeNH molecules finally decompose to yield HCeN + H2 upon photolysis. The periodic trend and differences for the M + NH3 (M = Ti, Zr, Hf, Ce, Th) systems are discussed on the basis of the present and previous works. DFT calculations predict that the most stable ground state for HHfNH2 and HThNH2 is singlet due to the stronger relativistic effects in Hf and Th atoms, while that for HTiNH2, HZrNH2, and HCeNH2 is triplet. Besides, the H2-elimination process is different for Ce and M (Ti, Zr, Hf, Th) cases.

Titanium (Ti) decoration is considered as a promising strategy for preventing metallic uranium (U) surface from oxidization and corrosion. The O adsorption and diffusion behaviors on the clean and Ti-decorated γ-U(0 0 1) surfaces have been investigated by density functional theory (DFT) calculations in the present work. The preferred sites and stable configurations for O atoms on the surface and in the subsurface have been evaluated. It is found that O atoms tend to adsorb on the surface instead of absorbing in the subsurface. With nudged elastic band method, O atom on Ti-doped surfaces departs from the energetically favored site which there is a higher barrier than it on clean U surface. O and Ti can form a chemical bond of Ti-O, reducing the interaction of U and O, thereby enhancing the corrosion resistance of U surface.

First-principles density functional theory are used to calculate the formation energies of ten light impurities X (X: H, He, Li, Be, B, C, N, O, F and Ne) in seven actinide dioxides AnO2 (An: Th, Pa, U, Np, Pu, Am and Cm), in order to elucidate the relative stability of X and to obtain some trends of impurities behaviors. The Hubbard parameter U is used to describe the strongly correlated electron behavior of An 5f electrons. The results indicate that the formation energies of X significantly depend on the properties of AnO2 and X. For X at the octahedral interstitial sites of AnO2, F is the only energetically favorable impurity for all AnO2, owing to its strong oxidability; H in PaO2, O in PaO2 and UO2, Li in PuO2, AmO2 and CmO2, Be in AmO2 and CmO2 are also energetically favorable. The oxidability or reductivity of X and the delocalization → localization transition of 5f electrons across actinide series can account for the trends of the behaviors of X in AnO2. Particularly, H, a very typical amphoteric element, is chosen to illustrate its difference existence states in AnO2. H prefers to occupy the octahedral interstitial sites of early AnO2 or form hydroxyl group in the later AnO2.

A systematic study of the phase stability, chemical bonding, and lattice dynamics properties of PuGaO3 is performed by means of unbiased particle-swarm optimization techniques combined with first-principles calculations. Phase stability analyses, including total energy, formation enthalpy, mechanical stability, and lattice dynamics, indicate that the ground-state structure of PuGaO3 at ambient pressure belongs to the space group Pnma. The fat-band dispersions and chemical bonding analysis suggest that Pnma PuGaO3 is an indirect band gap semiconductor (2.46 eV at the PBEsol + U level) at the Γ point and the Pu–O and Ga–O bonds in Pnma PuGaO3 are dominated by the ionic interactions. By examining the phonon spectra and phonon DOS, we provide a detailed insight into the lattice dynamics and thermodynamic properties of PuGaO3. In addition, the infrared absorption and Raman scattering spectra of PuGaO3 are predicted and further assigned at the center (Γ point) of the first Brillouin zone.

As B12 clusters exhibit significant structural stability due to double aromaticity, metal doped-B12 clusters often prefer a half sandwich structure. Herein, we report a systematic theoretical study on the geometric and electronic structures, and chemical bonding of the half sandwich AnB12 (An = Th to Cm) clusters to explore the stability and extent of covalency of the An–B bonds of these actinide borides. We have shown that in the gas-phase clusters, the significant stability of AnB12 is determined by electrostatic and orbital interactions between the An 5f6d7s orbitals and π-type molecular orbitals from B 2p orbitals of the B12 unit. A change-over of An–B bond length from An = Th to Cm is found at An = Pa as a result of actinide contraction combined with weakening An–B bonding due to an energy decrease and orbital localization of the 5f orbitals. Consistently, the oxidation states of the An atoms at first increase from Th(f0)IV to Pa(f0)V, and then due to the 5f-AO contraction, they smoothly decline to U(f2)IV, Np(f4)III and Pu(f5)III, and then eventually to Am(f7)II but Cm(f7)III, both with a half-filled 5f shell.

The hydrogenation of metallic plutonium (Pu) is a crucial issue for its actual applications. In-depth scientific understanding of the hydriding corrosion process of Pu requires established values of rates and associated microscopic mechanism of hydrogen diffusion in the reaction product plutonium hydrides. Therefore, in this work we investigate the hydrogen diffusion in the stoichiometric plutonium dihydride and the non-stoichiometric one that contains tetrahedral hydrogen vacancies from first principles. Our results reveal the microscopic dynamical behavior of hydrogen in these compounds and indicate that the dominant diffusion mechanism greatly depends on the temperature and composition.

Plutonium (Pu) can react with hydrogen to form complicated continuous solid solutions with unusual chemical and physical properties. The PBE0 hybrid density functional under the framework of full-potential linearized augmented plane wave plus local orbitals is employed to investigate the structural, magnetic, lattice vibrations, and thermodynamic properties of face-centered cubic plutonium hydride (PuH2+x, x = 0, 0.25, 0.5, 0.75, 1). The decreasing trend with increasing x of the optimized lattice parameters is in reasonable agreement with experimental findings. According to the calculated formation enthalpies of PuH2+x compounds, all PuHx for both ferromagnetic(FM) and antiferromagnetic (AFM) phase are thermodynamically favorable, and the FM phase plutonium hydride is more favorable than the AFM phases. The characteristic Raman-active and the infrared-active modes at the center (Γ point) of the first Brillouin zone were further assigned and discussed. Finally, the free energy F, internal energy E, vibration enthalpy S, and constant-volume specific heat CV of PuH2+x are calculated in the range of 0–1000 K.

The hydriding corrosion of metallic plutonium (Pu) is a crucial issue for its actual applications. The current work performs systematic ab initio molecular dynamics calculations to reveal the microscopic mechanism of Pu oxides over metallic Pu in resisting hydriding reaction. The results show that Pu hydriding is inhibited by PuO2 because hydrogen atoms are captured by lattice oxygen to form hydroxyl groups. In α-Pu2O3 the hydrogen attack could be prevented when hydrogen concentration is low because oxygen vacancy acts as a trap for hydrogen migration. Whereas, when the hydrogen concentration is high enough the hydriding corrosion could not be effectively resisted.

Uranium nitrides are one kind of accident-tolerant fuels and have been paid more attention recently. With the first-principles Density-Functional Theory (DFT) calculations, the adsorptions properties of some typical atoms, molecules and radical (including O, H, H2, O2, H2O and OH) adsorbed on the UN2 (001) surface have been studied in the present work. The preferred sites and stable configurations for those adsorbates on the UN2 (001) surface have been obtained. It's found that O or H atom prefers to be adsorbed at the bridge site; O2 adsorption will dissociate into two O atoms and occupy the nearest neighbor bridge sites; the interaction between H2 molecule and the UN2 (001) surface is very weak; OH prefers to occupy the bridge site with its O-H bond vertical to the surface; the surface adsorption of H2O is non-dissociated and adsorption energies are dependent on the initial structures and adsorption modes.

The formation and migration energies for various point defects, including vacancies and self-interstitials in aluminum are reinvestigated systematically using the supercell approximation in the framework of orbital-free density functional theory. In particular, the finite-size effects and the accuracy of various kinetic energy density functionals are examined.The calculated results suggest that the errors due to the finite-size effect decrease exponentially upon enlarging the supercell. It is noteworthy that the formation energies of self-interstitials converge much slower than that of vacancy. With carefully chosen kinetic energy density functionals, the calculated results agree quite well with the available experimental data and those obtained by Kohn-Sham density functional theory which has exact kinetic term.

Molecular dynamics simulations have been performed to investigate the structural evolution of Cu 64.5 Zr 35.5 metallic glasses under irradiation. The largest standard cluster analysis (LSCA) method was used to quantify the microstructure within the collision cascade regions. It is found that the majority of clusters within the collision cascade regions are full and defective icosahedrons. Not only the smaller structures (common neighbor subcluster) but also primary clusters greatly changed during the collision cascades; while most of these radiation damages self-recover quickly in the following quench states. These findings indicate the Cu-Zr metallic glasses have excellent irradiation-resistance properties.

Herein, an experimental study of the vibrational spectra of HCeN was carried out in solid argon, followed by theoretical investigations of molecular structures and the nature of CeN bond. The absorption band at 937.7 cm−1 with the 1.0311 14N/15N isotopic shift ratio is characteristic of CeN stretching band for HCeN, showing a 94 cm−1 higher shift relative to that of the diatomic CeN molecule. This large frequency shift indicates a much stronger CeN bond in HCeN, which is confirmed by DFT calculations. Qualitative orbital interaction and orbital composition analyses suggest that the addition of the H ligand to the Ce center will activate the 4f valence shell and strengthen the covalent bond between Ce and N, which may contribute to enhance the CeN triple bond in the HCeN molecule.

The metallization of neon remains a controversial problem as there is no consensus in theoretical simulations and no experimental verification. In this work, the insulator-to-metal transition in fcc solid neon at high pressure was revisited with a coupling of the all-electron full-potential linear augmented-plane wave (FP-LAPW) method and the GW correction to avoid the potential unreliability of pseudopotential under high pressure and correct the inaccurate energy gaps caused by local density or generalized gradient approximation of the exchange-correlation. This FP-LAPW + GW calculation predicts that the bandgap closes at a density of 88.3 g/cm3 and a pressure of 208.4 TPa. Moreover, the reported positive pressure dependency of energy gap (increases with increasing density) for solid neon in 1.5–10.0 g/cm3 was confirmed with our FP-LAPW calculations, and the underlying mechanism was first revealed based upon analysis of the charge density distribution and the electron localization function. The results of this research will provide a valuable reference for future high pressure experiments and shed new insight into the planetary interiors.

As impurities are virtually impossible to exclude from Pu oxides in realistic environments, understanding the roles of impurities is crucial for the applications and designs of Pu oxides. Here we perform a systematic first-principles DFT + U calculation to find the trends of transition-metal (TM) behaviors in PuO2 in terms of energetics, atomic properties, oxidation states, and electronic structures. The results show that group IV-B elements Ti, Zr, and Hf are energetically and electronically favorable in PuO2 and render the possibilities of forming Pu-TM-O ternary phases. In contrast, the remaining TMs tend to destabilize PuO2 and whether phase segregation or transition occurs largely depends on the redox conditions: oxidation one induces segregation, whereas reduction one facilitates the transition from PuO2 to Pu2O3. On the basis of the correlations between the properties of TMs and their relative stabilities in PuO2, we conclude that the degree of electron match between TMs and Pu plays the decisive role in the stability, as established for the cases of tetravalent elements, whereas some electron-mismatched but energetically stable TMs such as III-B and V-B elements could drive the valence transition of Pu, resulting in the phase instability of PuO2.

Hydrogen in metal oxide is a classic but engaging topic in the scientific research and actual application; owing to the flexible property of hydrogen, however, a scaling relationship regarding the existence state of hydrogen remains in lack. Hereby, a systematic first-principles calculation is performed on the behavior of neutrally charged hydrogen atom in a broad variety of metal binary oxides. Three main incorporation sites for hydrogen (interstitial site, hydroxyl group and oxygen vacancy) and more than 100 metal oxides are considered. From the calculated defect energy, the relatively stable incorporation sites for hydrogen are identified, and the mechanism behind the stability is unravelled by the inherent redox and electronic structure properties of host oxides. Generally, hydrogen prefers to form hydroxyl group in the reducible oxides but occupy interstital site in the irreducible ones. More importantly, oxygen vacancy formation energy associated with the redox property of oxide is established as the pragmatic descriptor for scaling the existence state of incorporated hydrogen, and an intercept value of about 4.5 eV is determined for distinguishing H occupancy, coincidencent with the standard hydrogen electrode potential. The scaling rule is expected to be popularized to other oxides and solids associated with hydrogen behavior.

Oxidation state (OS) is one of the most important concepts in chemistry and, the challenging task is how to rigorously determine the OS of multivalent solid ions of which uranium is the outstanding example. Herein, we revisit the OS of solid uranium ions in a rich variety of compounds by calculating density functional theory (DFT)-based OS (OSDFT). The results disclose the variations of uranium OSDFT with chemical surrounding and local structure; OSDFT is generally consistent with formal OS (OSf) in the ionic approximation for the majority of compounds. Interestingly, uranium OSDFT in some compounds containing uranyl or uranyl-like units, e.g., β-/γ-UO3, and UO2X2 (X = F, Cl, Br, OH), adopt U4+, instead of the textbook and never contradicted U6+ for gaseous and liquid uranyl ions. The associative effects from ligand-ligand interaction, steric repulsion, crystal-field splitting and delocalization-localization dual nature of 5f electrons are proposed to be responsible for the unusual OS behavior in solid-state uranium compounds.

We perform first-principles calculations of crystal structure, magnetism, electronic structure, chemical bonding and elastic properties for PuC2 using the standard local spin-density approximation (LSDA)+U scheme. The use of the Hubbard term to describe the 5f electrons of plutonium is discussed according to the lattice parameters, magnetism and densities of states. Our calculated lattice constants and magnetism are in good agreement with the experimental data or other theoretical calculations. It is shown that the total densities of states at the Fermi energy level mainly come from the contribution of narrow f band. The Pu-C bonds of PuC2 have a mixture of covalent character and ionic character, while covalent character is stronger than ionic character. The C1-C2 bonding has strong covalent character because of sp2 hybridization between C atoms. Lastly, the elastic properties of PuC2 are studied. We hope that our results can provide a useful reference for further theoretical and experimental research on PuC2.

First-principles density functional theory–generalized gradient approximation methods are used to calculate the energetics of Ga in Ga-stabilized δ phase Pu, Pu 2 O 3 and PuO 2 , in order to elucidate the relative stability of Ga in the Pu oxide layers. The Hubbard parameter U is used to describe the strongly correlated electron behavior of Pu 5 f electrons. Three incorporation sites for Ga, i.e., interstitial site, Pu and O vacancies, are considered. The results indicate that the energetics of Ga significantly depend on the inherent properties of the host materials and the incorporation sites in them. Ga incorporation into interstitial sites and O vacancies of both Pu 2 O 3 and PuO 2 are energetically unfavorable. Ga incorporation into Pu vacancy of PuO 2 is the most energetically favorable, followed by Ga incorporation into Pu vacancies of Pu 2 O 3 and Pu. However, this does not mean that there would be the highest concentration of Ga in PuO 2 among these three Ga-stabilized Pu substances because it is most difficult for Pu vacancy formation in PuO 2 . The distribution of Ga in the Pu oxide layers is proposed to be strongly dependent on the distribution and the concentration of Pu vacancy, as a result, Ga concentration likely decrease with the transitions of Pu → Pu 2 O 3 → PuO 2 .

We perform first principles calculations to investigate the structural, magnetic, electronic and optical properties of PuC and PuC0.75. Furthermore, we examine the influence of carbon non-stoichiometry on plutonium monocarbide. For the treatment of strongly correlated electrons, the hybrid density functionals like PBE0, Fock-0.25 are used and we compare the results with the generalized gradient approximation (GGA), local density approximation (LDA), LDA + U and experimental ones. The optimized lattice constant a0 = 4.961 A for PuC in the Fock-0.25 scheme is the most close to the experimental data. The ground states of PuC and PuC0.75 are found to be anti-ferromagnetic. Our results indicate that additional removal of a C atom make lattice contract and new DOS peak appear in the near-Fermi region. We also compute and compare the optical properties of PuC and PuC0.75. The difference in optical properties between PuC and PuC0.75 should also be the influence of carbon vacancies.

First-principles density functional theory–generalized gradient approximation methods have been used to calculate the energetics (incorporation energy, formation energy and binding energy) of rare gases (He, Ne, Ar, Kr and Xe) at the three incorporation sites (octahedral interstitial, uranium and oxygen vacancies) of uranium dioxide. The Hubbard parameter U and van der Waals corrections have been used to describe the strongly correlated electronic behavior of uranium 5f electrons and the weak interactions of rare gases, respectively. The results indicate that the energetics of rare gases depend significantly on the incorporation sites and on the atomic properties such as atomic radius. All rare gases considered here are energetically unfavorable at the three incorporation sites. However, rare gases exhibit significant binding ability to both U and O vacancies. The main trends of relative stability of rare gases generally reflect a size effect: the rare gases become more unstable with increasing atomic number. Electronic structures of these systems containing rare gases also exhibit general trends in their relative stability and charge-transfer character.

As the plutonium (Pu) oxides are the crucial issue of the Pu surface and corrosion science and some impurities like hydrogen (H) are virtually impossible to exclude from them in realistic environments, understanding the dynamical behaviors of hydrogen (H) in Pu oxides is essential for the protections and applications of Pu. Here we perform systematic ab initio molecular dynamics (AIMD) calculations to reveal the diffusion behaviors of impurity H in the oxygen saturated (OS) and oxygen unsaturated (OU) Pu dioxide (PuO2). The considered variation ranges of the concentration of interstitial H (θ: H to Pu ratio), the concentration of oxygen vacancy (OV) (nov: OV to O ratio), and the temperature were 1 32 ∼ 14 32 , 0 ∼ 8 64 , and 300 ∼ 800 K, respectively. The root mean square displacements, radial distribution functions, Bader charge, electronic density of states, and the three-dimensional diffusion coefficients were extracted and analyzed in detail. Results indicate that the H diffusion is inhibited mainly due to the formation of hydroxyl with the host O both in the OS and OU PuO2. In the OU PuO2 the O atoms will transport due to the presence of large concentration of H. The octahedral interstitial site in the OS PuO2 and the OV in the OU PuO2 provide a metastable and favorable capturing effect to the interstitial H, respectively. The diffusion coefficient in the OU PuO2 is smaller than that of in the OS PuO2 and decreases with nov essentially as the OV plays the role of a trap for H diffusion. Our conclusions may provide potential guidance for Pu surface and corrosion science.

Density functional theory calculations were performed to study reactions on uranium mononitride (UN) surface when it is in contact with environment gases such as O2, H2O, N2, H2. The chemical adsorption of O2 on UN surface is much more active than other surface reactions. Microkinetic analysis shows that the UN surface is mainly adsorbed by N2 at room temperature, suggesting nitrogen molecules in the air can protect UN from being oxidized. When temperature is above 350 K, the rate of surface oxidation speeds up significantly with the full O-adsorption on UN occurring within a relatively short time. Ab initio thermodynamic calculations display that the UN surface undergoes atomic rearrangements upon oxygen adsorption, which leads to the expansion of crystal lattice and opens the pathways for O diffusion into the sub-surface. Our work constitutes the first ab initio-based kinetic study of surface reactions on UN and contributes to the understanding of oxidation corrosion of this promising nuclear fuel at atomic scales.

We investigate the distribution preference of hydrogen in the α-U/UO2 interface by using the DFT+U method. We find that a monolayer of hydrogen atoms firstly assembles right in the interface between α-U and UO2. Through detailed electronic state analysis, it is revealed that the formation of U-H bonds with uranium atoms at the UO2 side favors this hydrogen atom assembly. The newly formed U-H bonds are similar to the U-O-U superexchange interactions in UO2. The incorporation of hydrogen in either α-U or UO2 is much higher in energy. After formation of two hydrogen monolayers, following hydrogen atoms have no such preference in the interface area, and tend to distribute inside α-U.

The nature and extent of the highest oxidation state (HOS) in solid-state actinide compounds are still unexplored compared with those of small molecules, and there is burgeoning interest in studying the actinide–ligand bonding nature in the condensed state. A comprehensive understanding of the electronic structure and unraveling the possibility of a HOS are of paramount importance in solid-state actinide chemistry. Here, we report the physical OS of the early to middle actinides (Th → Cm) in solid-state compounds via a more rigorous quantum mechanical definition of OS under the DFT+U theoretical frameworks for the first time. This work implies that the highest physical OS of the Pu solid ion is PuV in PuO2F and PuOF4, which can be achieved via tuning the ligand, thus improving our knowledge of oxidation states and chemical bonding in high OS solid-state compounds. We highlight the importance of ligand design in terms of the actinide HOS, employing a highly electronegative ligand and showing the capacity to form multiple bonds.

As B12 clusters exhibit significant structural stability due to double aromaticity, metal doped-B12 clusters often prefer a half sandwich structure. Herein, we report a systematic theoretical study on the geometric and electronic structures, and chemical bonding of the half sandwich AnB12 (An = Th to Cm) clusters to explore the stability and extent of covalency of the An-B bonds of these actinide borides. We have shown that in the gas-phase clusters, the significant stability of AnB12 is determined by electrostatic and orbital interactions between the An 5f6d7s orbitals and π-type molecular orbitals from B 2p orbitals of the B12 unit. A change-over of An-B bond length from An = Th to Cm is found at An = Pa as a result of actinide contraction combined with weakening An-B bonding due to an energy decrease and orbital localization of the 5f orbitals. Consistently, the oxidation states of the An atoms at first increase from Th(f0)IV to Pa(f0)V, and then due to the 5f-AO contraction, they smoothly decline to U(f2)IV, Np(f4)III and Pu(f5)III, and then eventually to Am(f7)II but Cm(f7)III, both with a half-filled 5f shell.

Correction for 'Insights into the enhanced Ce[triple bond, length as m-dash]N triple bond in the HCe[triple bond, length as m-dash]N molecule' by Zhen Pu et al., Phys. Chem. Chem. Phys., 2017, 19, 8216-8222.

The effective on-site Coulomb interaction (Hubbard $U$) between 5$f$ electrons in actinide metals (Th-Cf) is calculated with the framework of density-functional theory (DFT) using linear response approach. The $U$ values seldom rely on the exchange-correlation functional, spin-orbital coupling, and magnetic states, but depend on the lattice volume and actinide element. Along the actinide series, the Coulomb parameter $U$ of $\alpha$-phase first decreases slowly, followed by a jump in the vicinity of Pu and then a monotonous increase. For light actinides, the lattice volume has a sizeable influence on $U$ while the localization of 5$f$ electrons is almost constant. But for transplutonium metals, $U$ is almost independent of the lattice volume but the electronic localization increases rapidly. The calculated lattice parameters from DFT+$U$ with the Coulomb parameters as input are in better agreement with the experimental values than those from DFT within local density approximation or Perdew-Burke-Ernzerhof approximation for solids (PBEsol). In particular, the agreement between PBEsol+$U$ and experiment is remarkable. We show that PBEsol+$U$ also well reproduce the experimental bulk moduli and the transition from itinerancy to localization of 5$f$ electrons along the series. Therefore it is concluded that DFT+$U$ with $U$ calculated from linear response approach is suitable for a good description of actinide metals., Comment: 10 pages, 9 figures