Unraveling the chemical behavior of actinide species is difficult owing to the complex electronic structure of these species, the fact that many of these species can occur in multiple oxidation states and complex condensed phase structures, and the difficulties encountered in their experimental studies. By using first principles models, we can begin to unravel the chemical behavior of actinide species. Here, schemes for modeling the hydration shell structure, x-ray absorption spectra, hydrolysis and solvent exchange energetics of actinide complexes in solution will be presented. We have been able to accurately simulate the EXAFS spectra for aqueous actinides in different oxidation states including U(VI), U(IV), and Cm(III) using relativistic ab initio molecular dynamics methods. We also probed the thermodynamics of hydrolysis by calculating the first acidity constant for uranium in all three oxidation states (IV, V, and VI). We predicted, for the first time, that UO2+ is a weak acid in solution with a pKa value of 8.5, which is unexpectedly small for a +1 ion. This result is particularly important since no thermodynamic data are available for hydrolyzed species of U(V). In our most recent work on concentrated Cm(III) solutions, we showed that counter-ions can strengthen or weaken the solvent structure itself rather than just the water coordination number. These new results are better explained in terms of the hydrogen bond lifetimes in the solvent shell being affected by the polarization due to the counter-ions.

Uranium exhibits two types of the hydride UH₃. α-UH₃ formed at low temperatures is rather unstable and could never be studied in a pure form. It transforms easily into β-UH₃, known as a band ferromagnet with T_C asymp; 170 K. We have been testing the H reaction with bcc γ-U alloys stabilized to room temperature by Mo or Zr doping in conjunction with ultrafast cooling. We have found that both types of alloys react with H₂ only at elevatad pressures (min. 4-5 bar). The alloys with Mo yield brittle but compact hydrides UH₃Mo_x, which allow to measure the temperature dependence of electrical resistivity and specific heat. These quantities together with magnetization measurements indicate that the T_C values are enhanced, reaching 200 K for UH₃Mo_0.18 [1], while for higher and lower Mo concetrations the value tends to decrease. XRD pattern indicates an amorphous structure, which can be interpreted as based on the cubic β-UH₃ structure with grain size about 1 nm. Magnetic moments per U atom are practically independent of the Mo concentration, and are about 20% higher than those of β-UH₃ (asymp; 0.9 mu;_B). Probably due to similar atomic size of U and Zr, the amorphization does not happen in the case of UH₃Zr_x; the structure corresponds to bcc, i.e. to γ-U expanded by embedded H. It is in fact the crystal structure of α-UH₃. Its lattice parameter remains aboout 1 % below 416 pm of α-UH₃. The U-U spacing of 358 pm is much higher than that in β-UH₃ (330 pm). Therefore it is quite surprizing that both T_C and U moments are very similar to those in β-UH₃. These results help to solve an old issue of the magnetic state of α-UH₃. The fact that samples containing mixed α and β phase exhibited only one ferromagnetic transition was sometimes interpreted as due to the non-magnetic character of α-UH₃. This work was supported by the Czech Science Foundation under the grant P204/12/0285. [1] I. Tkach et al. PRB 88, 060408 (2013).

Density functional theory (DFT) for plutonium metal is shown to be consistent with recent magnetic measurements that suggest anti-ferromagnetism in Pu-Ga alloys at low temperatures. The theoretical model predicts a stabilization of the face-centered-cubic (fcc, δ) form of plutonium in an anti-ferromagnetic configuration when alloyed with gallium. The ordered magnetic phase occurs because Ga removes the mechanical instability that exists for unalloyed δ-Pu. The cause of the Ga-induced stabilization is a combination of a lowering of the band (kinetic) and electrostatic (Coulomb) energies for the cubic relative to the tetragonal phase. Similarly, gallium plays an important role in stabilizing anti-ferromagnetism in the tetragonal P4/mmm Pu3Ga compound. This work was performed under the auspices of the U.S. DOE by LLNL under Contract DE-AC52-07NA27344.

The response of actinide materials to radiation is of critical importance to their application in fission energy systems, with respect to both their performance as nuclear fuels and their environmental behavior. Through fission and nuclear decay, such materials are exposed to ionizing radiation by alpha particles, beta particles, and energetic fission fragments, all of which interact with matter primarily through electronic excitation. In contrast to neutron irradiation, which produces structural damage via atomic collisions, the energy deposition from these particles is mediated by the electronic structure of the target material and is therefore strongly affected by the complex chemistry of actinides. Due to the partially delocalized nature of their 5f electrons, the light actinides show large variability in their oxidation states in ionic compounds, with the exception of thorium, which features no f-electrons and strongly favors a tetravalent configuration. This redox variability leads to unique behavior during the relaxation of the excited electron cascades produced by highly ionizing radiation.
We have irradiated various actinide materials with highly ionizing radiation, in the form of heavy ions accelerated to relativistic velocities, and characterized the resulting modifications using complementary synchrotron x-ray diffraction (XRD) and absorption spectroscopy (XAS) techniques. This combined structural and chemical analysis reveals coupling of atomic structure modifications to modifications of the electronic structure, in that much of the damage produced in actinide materials results from radiation-induced redox processes. Following irradiation, reduction of the valence of actinide cations is observed, causing structural distortion or phase transformations due to the concomitant changes in ionic radius and coordination chemistry. We present results for ThO₂, the actinide analogue CeO₂, with cerium's single 4f electron yielding valence variability mimicking that caused by the itinerant 5f electrons in the light actinides, and the hexavalent uranyl compounds (UO₂)(OH)₂ and (UO₂)₈O₂(OH)₁₂(H₂O)₁₀. The potential extent of cation reduction and, therefore, susceptibility to radiation damage, increases from thorium (stable only in a 4+ valence in oxides) to cerium (3+, 4+) and uranium (4+, 5+, and 6+). Thus, radiation tolerant materials can be designed by limiting the extent or efficiency of oxidation state reduction in actinides. These results are of particular significance to the use of nanomaterials in nuclear reactors, as they show enhanced redox potentials and susceptibility to radiation damage, and to the reactivity of actinide materials in both nuclear reactors and the environment.

The formation enthalpies and defect ordering tendencies for rare earth trivalent fission products in uranium dioxide are calculated using atomic-scale simulations. Low-energy defect structures are first screened with calculations based on core-shell ionic-potential models. The most stable defect structures are then studied by density functional theory (DFT) with Hubbard-U (DFT+U) corrections. The calculations consider compositions in which the trivalent cations are charge-compensated by oxygen vacancies and holes. Calculated formation enthalpies and cation ordering tendencies for vacancy-compensated systems are compared with calculations of relevant trivalent doped fluorite systems, i.e., yttria-stabilized zirconia and yttria-doped thoria.

Due to the complexity of 5f electron behavior and variable oxidation states of uranium, the phase diagram of U-O binary system is very complicated. Variation in U oxidation state leads to different stoichiometries, and many different compounds are formed. Even the same composition, UO2+x has many different polymorphs. The structural stability of uranium oxides is very important to their use as a nuclear material, particularly nuclear fuels. Structural behavior of U3O8 compound was studied here by in situ XRD at high pressure and high temperature conditions.
U3O8 was obtained by annealing UO3 in a reducing atmosphere at 200 °C. After 10 hours heating, the yellowish UO3 changed to a dark colored powder. Both XRD and Raman measurements confirmed that the obtained powder after annealing is in the β-U3O8 structure (orthorhombic, C222, with lattice parameters: a = 6,742 (3) Å, b = 11.948 (4) Å and c = 4.1428(6) Å). A powder sample of β-U3O8 was pressurized at room temperature up to 37.5 GPa with a symmetric diamond anvil cell. XRD patterns clearly indicated that a phase transition occurred between 3-11 GPa. The high-pressure phase is a fluorite-like structure. The fluorite-like structure is stable up to 37.5 GPa. The high-pressure phase was then laser heated to over 1700 K in the diamond anvil cell at high pressure conditions. The diffraction peaks became sharper due to the high temperature annealing. No phase transition was found at high pressure/ temperature conditions, and the fluorite-like structure of U3O8 is even fully quenchable. The lattice parameter of the fluorite-like high-pressure phase is 5.425 Å at ambient conditions, which is smaller than that of the stoichiometric UO2.
Previous experiments have shown that the stoichiometric uranium dioxide (UO2) is not stable at high pressure conditions and starts to transform to a cotunnite structure at ~30 GPa. When heating the sample at high pressure, the critical transtion pressure is greatly reduced. However, the fluorite-like high-pressure phase of U3O8 is very stable at high pressure/high temperature conditions. The enhanced phase stability is believed to be related to the presence of extra oxygen (or U vacancies) in the structure.

Y3Fe5O12 (YIG) garnet is a potential nuclear waste storage material due to its ability of incorporating various actinide elements. Since cerium (Ce) can be considered an analogue of actinide elements such as plutonium (Pu) and uranium (U), studying the local structures, charge states and thermodynamic stability of Ce-substituted YIG (Ce:YIG) garnet is useful to design stable and efficient actinide substitution in the garnet system. In experiments, YIG with Ce substitution amounts ranging from x = 10 to 25 mol % (Y3-xCexFe5O12) were synthesized and analyzed. Ce was found to be in the trivalent state at low substitution levels and mixed-charge states with trivalent and tetravalent states at higher concentrations. A thermodynamic analysis suggests that the substitution reaction is not enthalpically favorable but entropically driven. A series of GGA+U calculations have been undertaken to study Ce substitution in YIG. The calculations confirm that Ce4+ is not stable at low Ce content, but can be stabilized at higher concentrations, where charge transfer occurs between the trivalent Ce at dodecahedral sites and trivalent Fe at tetrahedral sites. Detailed analyses of atomic structures suggest that the increasing stability of Ce4+ is associated with the increasing strain energy originating from the size differences between Ce3+ and Y3+. The calculated reaction energies qualitatively agree with the experimental measurements, confirming that both Ce substitution and Ce-Fe charge transfer are weak endothermic reactions driven by entropic effects.

The main application for uranium dioxide microspheres, either in pure form or doped with other oxides, is as fuel in a high-temperature gas-cooled reactor. They can be compacted through low-energy vibration or densified by routine pressing and sintering. These microspheres can be fabricated only by sol-gel processes. We first used the classical variant of processing, as elaborated at Oak Ridge National Laboratory, consisting of (1) reduction of commercial uranyl to U(IV) nitrate; (2) preparation of sol by (a) precipitation peptization of uranium hydroxide (b) solvent extraction of nitrates to prepare concentrated (>1 M) urania sols; and (3) gelation to microspheres by extraction of water by dewatered 2-ethyl-1-hexanol (2EH) emulsion added to the solvent. We incorporated Pt/Al2O3 catalyst spheres in step 1, which induced formation of a U(IV) solution that was free of NH4+. This ion is highly deleterious to the process if the step 2b variant is followed. In step 3, we improved formation of the emulsion by application of vibrating capillary feeding of the sols and radically decreasing the dewatering temperature by use of a vacuum. Substantial improvement in microspheres production was achieved by application of a novel sol-gel process, which was first used to prepare high-temperature superconductors (Complex Sol-Gel Process, Polish Patent No. 172618 (1997)) and has since been applied to preparation of various forms of a wide range of complex oxides. In this method, the reduction step was omitted and uranyl (VI) ascorbate sols/hydroxyl sols were formed from suspension of either a uranium trioxide or a uranyl nitrate solution. A second extraction of water with nitrates proved to be necessary to produce perfectly spherical microspheres that were equivalent to those produced in the method from Oak Ridge. Thermal treatment by calcination and reduction in hydrogen atmosphere, which was designed on the basis of differential thermal analysis and thermogravimetric analysis studies, yielded in high-density microspheres of uranium dioxide powders. We also demonstrated that doping of surrogates of minor actinides (e.g., Nd) into uranium dioxide powders could be easily synthesized by the Complex Sol-Gel Process.

Zirconolite is one of the component of Synroc materials which have been regarded as the second generation of high level waste (HLW) forms in the world. It allows to incorporate into the crystal structures almost all of the elements present in high-level radioactive waste. The leach resistance and physical properties this kind of ceramic is suitable for HLW immobilization.
Zirconolite (CaZrTi2O7) immobilizes mainly strontium (Sr) and barium (Ba) but also allows to immobilize in its structure long-lived actinides such as plutonium (Pu). Zirconolite phase has been fabricated by a sol-gel route using original method called Complex Sol Gel Process (CSGP) (method developed for synthesis Me- titanates materials Polish Patent PL 198039, 2001). The process was adapted to synthesis of zirconolite for nuclear waste immobilization. Into the structure of the material were incorporated non-radioactive surrogates of nuclear waste such a Sr, Co, Cs and Nd (in the molar ratio 10%). The samples was determined by thermogravimetric analyses and X-ray diffraction. Studies indicated that transformation of ascorbate- nitrate gels (CSGP) to dopped zirconolite phases is definitely lower that gels without ascorbic acid addition.

Fabrication of thin films is an indispensable part for various fundamental studies of materials properties. The thin film approach has also made its way into the actinide materials research area. There are a great deal of publications on thin films studies of uranium and its compounds. Thin films of other actinides are less commonly studied. In this report we will present experimental results on depleted uranium oxide (UO2) thin film fabrication by electron beam evaporation and ion beam assisted deposition (IBAD) methods. The IBAD method, consisting of electron beam evaporation combined with low energy (1keV) Xe+ ion bombardment, was employed for controllable incorporation of Xe atoms in the UO2 matrix. The goal of this work was to fabricate and characterize reference samples to support development and validation of fission gas behavior models for predictive nuclear fuel performance codes. An effect of elevated deposition and annealing temperatures on the film&’s microstructure, Xe diffusion in UO2, and U diffusion into the silicon carbide substrate will be presented and related to the effects observed during nuclear fuel utilization. In addition, fabrication of well oriented UO2 films, with the microstructure approaching single crystalline quality, will also be demonstrated. Reported results will lend further support to application of actinide thin films for systematic studies elucidating fundamental properties of actinide materials.

In this talk theoretical calculations of ground state properties of actinide and lanthanide systems will be presented. Examples of properties that will be discussed are magnetism, cohesion, structural stability and finite temperature effects on the phase stability. The effect of correlation effects beyond that included in the local density approximation and generalized gradient approximation will also be discussed, in particular in light of dynamical mean field theory (DMFT). Examples of calculated properties with DMFT will be given, both ground state properties as well as spectroscopic data, and it will be argued that DMFT is important for an accurate description of spectra of lanthanide and actinide systems.

The combination of density functional theory in local density approximation and dynamical mean field theory (LDA+DMFT) was employed in a detailed study of the strong electron correlation effects in a promising nuclear fuel—uranium mono nitride (UN). For the ferromagnetic phase, the effective impurity problem in the LDA+DMFT [1-3] cycle is solved with the spin-polarized T-matrix fluctuation exchange (SPTF) solver, which includes spin-orbit interactions. Concerning the paramagnetic phase, the disordered local moment (DLM) approach was used, based on both basic LDA and LDA+U. Basic material properties (the spin, orbital and total magnetic moments on U atoms, and the density of states) were calculated for various values of the Hubbard parameter U with a fixed exchange parameter J. Our main focus was to compare the calculated spectral function (density of states) for different magnetic phases and different methods to the experimental XPS [4]. On top of that, the total moment from PM and FM are compared with the measured values by neutron spectroscopy [5].
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Density-functional theory (DFT) simulations are applied to obtain elastic, strength, and equation-of-state properties of actinide metals under extreme conditions. In this presentation, we will focus on pressure and temperature effects on the solid and liquid phases of uranium (U) metal.
For low temperature U metal, elastic constants are calculated directly from the DFT total energy for the ground-state phase in a wide pressure range. For the high temperature γ phase, we are applying a recent scheme to compute temperature-dependent phonon dispersions through the self-consistent ab initio lattice dynamics (SCAILD) technique. This approach is particular important for the high temperature phase (γ) where the elasticity cannot be analogously obtained because of its mechanical instability at lower temperatures. From these SCAILD phonon dispersions we then extract the elastic constants from the slopes approaching the Γ point. This technique has also been utilized successfully for other metals such as molybdenum. In addition, quantum molecular dynamics (QMD) calculations have been carried out for liquid U, particularly near the region of melt curve. We will discuss the ramification of QMD results on our development of equation-of-state and strength models.

Isotopomer separation of UF₆ based on different absorption properties of ²³⁵U and ²³⁸U containing molecules is promising economically and is reaching industrial scale (e.g. the SILEX process). Specifically, separation based on differences in vibrational spectra is enticing, as highly coherent infrared lasers become more and more affordable. Porting this process from the gas phase to a 2D adsorbate system is interesting, as it would make possible higher concentrations of UF₆ on an adsorbing surface and a smaller volume to be filled with laser radiation.
We present a computational study of UF₆ adsorption on graphene, doped graphene, and graphane. We show that (i) the isotopic splitting in the vibrational spectrum of UF₆ observed in vacuum is largely preserved in the adsorbed molecule; (ii) the adsorption energy of UF₆ is of the order of 1 eV on graphene and can be varied within half an eV by doping or surface functionalization, i.e. the adsorption strength is moderate and can be controlled by the choice of graphene derivative. (i) and (ii) mean that it may be possible to cause desorption of a selected isotopomer by laser radiation, leading to isotopic separation between the surface and the gas.

I present first-principles-based multiband spin-fluctuation calculations within the random-phase approximation of four isostructural intermetallic actinides, namely the superconductors PuCoIn5, PuCoGa5, PuRhGa5, and the paramagnet UCoGa5. The results show that a strong peak in the spin-fluctuation dressed self-energy is present around 0.5 eV in all materials, which is mostly created by 5f electrons. These fluctuations are coupled to electrons, which gives rise to the peak-dip-hump structure in the spectral function, characteristic of the coexistence of itinerant and localized electronic states. These results are in quantitative agreement with available photoelectron spectra on PuCoGa5 [1] and UCoGa5 [2].
The self-consistent intermediate Coulomb coupling GW calculations of the self-energy is performed within the fluctuation exchange approximation [3,4] using first-principles electronic structure input obtained from the density functional theory within the generalized gradient approximation (GGA). Notably, the effective coupling of electrons to spin fluctuations creates a dip in the single-particle excitations due to strong scattering between spin-orbit split states. The lost spectral weight (dip) in the spectral function is distributed partially to the renormalized itinerant states at the Fermi level (peak), as well as to the strongly localized incoherent states at higher energy (hump). The coherent states at the Fermi level can still be characterized as Bloch waves, though strongly renormalized, whereas the incoherent electrons are localized in real space exhibiting the dispersionless hump structure.
I will discuss the impact of the first-principles-based intermediate Coulomb coupling model for calculating electronic hot spots as material-specific markers in the spectral function, validation through experiments in UCoGa5, and the effects of multiband spin fluctuations as glue for superconductivity.
This work was supported by the U.S. DOE under Contract No. DE-AC52-06NA25396 through the Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, and the LDRD Program at LANL. I acknowledge a NERSC computing allocation of the U.S. DOE under Contract No. DE-AC02-05CH11231.
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The Hund's rule exchange interaction promotes a second-order phase transition to a coupled spin and orbital density wave state in the underscreened Anderson Lattice Model. The spin-flip part of the Hund's rule coupling stabilizes a spontaneous spin-dependent mixing of 5f quasiparticle bands which, in the normal state, have pure orbital characters. The transition breaks the spin-rotational invariance and leads to an asymmetric pseudo-gap forming in the density of states. When a magnetic field is applied, the electronic dispersion relations become dependent on the relative orientation of the field and the spontaneously chosen quantization axis. We show that this results in the magnetic susceptibility becoming anisotropic below the critical temperature, but without the development of a static magnetization

Nature of the so-called 'Hidden Order (HO)' of URu2Si2 has been a long-standing mystery in the heavy-fermion physics [1-3], invoking more than a few tens of exotic theoretical models [4]. Despite a clear evidence of a second-order phase transition seen in specific heat and other bulk quantities, no clear indication for the broken symmetry has been detected in microscopic measurements such as neutron and X-ray diffraction. Interestingly, broken four-fold rotational symmetry has recently been reported on the basis of magnetic-torque [5] and cyclotron-resonance measurements [6]. This has then raised a new argument for a possible "nematic order" of correlated electrons in HO. I will present here our latest tests to confirm microscopically the broken four-fold symmetry of this system. One of such attempts is a synchrotron-X-ray diffraction study performed to check the tetragonal-to-orthorhombic distortion that should be present in the nematic state. Our experimental data taken by using a high-resolution backscattering setup with a scattering angle of 2 theta = 179.81 degrees, however, have revealed that the tetragonal symmetry is conserved in HO within the experimental accuracy for the orthorhombicity (b' - a')/(b' + a') of ~ 1.0 x 10^-5. This indicates that the electron-lattice coupling in the nematic state is extremely weak, or simply that the four-fold symmetry is not broken. Present status and possible solutions of this long-lasting issue will be discussed.
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[4] J.A. Mydosh and P.M. Oppeneer, Rev. Mod. Phys. 83, 1301 (2011).

We propose a non-local exchange and correlation correction for f electrons to the standard local approximations for density functional theory calculations. The method is derived from configuration interaction of strongly correlated states rather than from the single-determinant Hartree Fock theory, therefore taking into account non-local static correlation. The many-body f-electron physics is treated explicitly by mapping the local f-electron charge density to a impurity problem.

This contribution will provide a brief overview of applications of advanced X-ray spectroscopic techniques that take advantage of the resonant inelastic X-ray scattering (RIXS) in the hard and tender X-ray range and have recently become available for studying the electronic structure of actinides^[1,2]. We will focus on the high-energy-resolution X-ray absorption near edge structure (XANES) and core-to-core and core-to-valence RIXS spectroscopies^[3] at the U L3 and M4,5 edges of uranium compounds^[1,2,4,5]. The experimental spectral features has been analyzed using a number of theoretical methods, such as the density functional theory in the local density approximation with an added Coulomb interaction (LDA+U), full multiple scattering (FEFF) and ab-initio finite difference method near-edge structure (FDMNES) codes. In connection with presented results, the capabilities and limitations of the experimental techniques and theoretical methods will be discussed.
References:
[1] K. O. Kvashnina, S. M. Butorin, P. Martin, P. Glatzel, http://arxiv.org/abs/1310.4003 2013.
[2] K. O. Kvashnina, S. M. Butorin, http://arxiv.org/abs/1310.4004 2013.
[3] P. Glatzel, T.-C. Weng, K. Kvashnina, J. Swarbrick, M. Sikora, E. Gallo, N. Smolentsev, R. A. Mori, J. Electron Spectros. Relat. Phenomena 2013, 188, 17-25.
[4] T. Vitova, M. A. Denecke, J. Göttlicher, K. Jorissen, J. J. Kas, K. Kvashnina, T. Prüssmann, J. J. Rehr, J. Rothe, J. Phys. Conf. Ser. 2013, 430, 012117.
[5] T. Vitova, K. Kvashnina, G. Nocton, G. Sukharina, M. Denecke, S. Butorin, M. Mazzanti, R. Caciuffo, A. Soldatov, T. Behrends, et al., Phys. Rev. B 2010, 82, 235118.

Recently, X-ray absorption spectroscopy (XAS) and related derivative measurements have been used to demonstrate the Pu 5f states are strongly relativistic and have a 5f occupation number near 5. [1] Owing to the success in this regime, it has been argued that the XAS measurements should be a powerful tool to probe 5f occupation variation, both as a function of elemental nature (actinide atomic number) and as a function of physical and chemical perturbation, e.g. oxidation state. It will be shown here that XAS and its related measurements fail in this latter aspect for a wide variety of uranium compounds and materials. Possible causes will be discussed.
Lawrence Livermore National Laboratory is operated by Lawrence Livermore National Security, LLC, for the U.S. Department of Energy, National Nuclear Security Administration under Contract DE-AC52-07NA27344. This work was supported by the DOE Office of Science, Office of Basic Energy Science, Division of Materials Science and Engineering. JGT wishes to thank Professor Guenter Kaindl for his critical reading of the manuscript and use of his XAS data.
1. J.G. Tobin, P. Söderlind, A. Landa, K.T. Moore, A.J. Schwartz, B.W. Chung, M.A. Wall, J.M. Wills, R.G. Haire, and A.L. Kutepov, J. Phys. Cond. Matter 20, 125204 (2008), and references therein.

The synthesis and study of soft-donor uranyl complexes can provide new insights into the coordination chemistry of non-aqueous {UO2}2+ leading to a better understanding of reactivity, bonding and structure within uranyl complexes outside of traditional oxygen-donor systems. Recently, the tunable N-donor ligand 2,6-Bis(2-benzimidazyl)pyridine (BBP) was employed to produce novel uranyl complexes in which the {UO2}2+ cation is ligated by anionic and covalent groups with discrete chemical differences. In this work we investigate the electronic structure of the three uranyl-BBP complexes [(UO2)(H2BBP)Cl2], [(UO2)(HBBP)(NC5H5)Cl], and [(UO2)(BBP)(NC5H5)2] via near-edge X-ray absorption fine structure (NEXAFS) experiments, density functional theory-based ground state total energy calculations, and NEXAFS simulations using the excited electron and core-hole (XCH) approach [1]. The evolution of the structural as well as electronic properties across the three complexes as a result of changing the coordination around the uranium site is studied systematically. The accuracy of a DFT based description of these f-electron coordination complexes is ascertained via test calculations employing beyond-GGA approaches such as self-consistent DFT+U [2], hybrid-DFT and perturbative GW corrections. Total energy calculations provide insight into the observed variations in structural properties across the three complexes, showing that the non-planarity in the equatorial co-ordination plane of Uranium is driven by steric effects. Computed N K-edge and O K-edge NEXAFS spectra are compared with experiment and spectral features assigned to specific electronic transitions in these complexes. The role played by residual solvent molecules within crystalline samples in modulating the spectra is also investigated. Furthermore, studying the variations in the energies of specific spectral features arising from N K-edge absorption provides a clear picture of ligand-uranyl charge transfer in these systems.
Acknowledgements:
This work was supported by the Laboratory Directed Research and Development Program of Lawrence Berkeley National Laboratory under U.S. Department of Energy Contract No. DE-AC02-05CH11231
References:
[1] D. Prendergast and G. Galli, X-ray absorption spectra of water from first-principles calculations, Phys. Rev. Lett. 96, 215502 (2006).
[2] M. Cococcioni et al., Linear response approach to the calculation of the effective interaction parameters in the LDA+U method, Phys. Rev. B 71, 035105 (2005).

Synchrotron radiation (SR) methods have been employed to investigate the chemistry and physics of a wide range of topics in actinide science for several decades. Over this period, the number of SR techniques brought to bear on critical issues in the actinide science field has grown greatly. Several of the results from actinide SR studies have yielded truly unique information that has fundamentally improved the understanding of actinide chemistry and physics. Recently, there have been increasingly significant activities in the soft X-ray region of the SR spectrum to probe the electronic structure of actinide materials. A specific emphasis has been on actinide complexes with light atom constituents that can be probed by near-edge X-ray absorption spectroscopy at the K-edge to yield quantitative bonding information. As part of the actinide soft X-ray efforts, a spectromicroscopy approach has been developed over the past decade using a scanning transmission X-ray microscope (STXM) at the Molecular Environmental Science (MES) Beamline 11.0.2 STXM of the Advanced Light Source. As the initial STXM approaches used for conducting actinide science investigations have matured, new innovations in STXM spectromicroscopy have promise to greatly expand the scope and breadth of actinide science studies that can be done with a soft X-ray STXM. This presentation will briefly recount recent progress in the soft X-ray region using absorption and emission techniques to set the stage for the focus on emerging scientific opportunities in the soft X-ray region that will not be limited to STXM alone. Additionally, the advent of new developments in light sources and detector technologies, present some unique opportunities for actinide science that will be highlighted.

Cast pucks and rods of high density alpha plutonium were created as a part of R&D efforts at Los Alamos National Laboratories. The as-cast material was analyzed using a variety of characterization techniques, including immersion density, resonant ultrasound spectroscopy (RUS), dilatometry, and nonlinear resonant ultrasound spectroscopy (NRUS). This talk will present the elastic moduli of alpha plutonium alloys measured in the as-cast and thermally treated state. Over 30 samples from three separate castings were measured. Results are presented as a function of density, alloy content and age. Comparisons between recent measurements at room temperature and elevated temperature will be compared to literature values and similar measurements on older alpha plutonium samples. Preliminary NRUS studies, used as a quality measurement on cast rods, will be presented as a function of casting conditions.

The various phase transformations in pure plutonium are characterized by large volume changes (+9 % to -2 %) and large temperature hystereses (10-100 °C) of the forward and reverse transformations. Details of the transformation mechanisms have proven elusive due to the difficulty of direct observation, but some appear to be martensitic (e.g., bc monoclinic β → monoclinic α) and others appear to be diffusional (e.g., fcc δ → bcc ε). One transformation that has particularly interesting behavior is fcc δ harr; orthorhombic γ, which bursts during cooling and is continuous during heating. In this talk, we will review dilatometric and calorimetric features of this transformation in various types of pure plutonium. These data are discussed in the context of the crystallography of the two phases and topological modeling of the δ/γ interface.

Plutonium is the most unusual element of the periodic table, highly unstable, presenting six allotropes from room temperature to its melting point, and of high interest due to its applications for nuclear energy and explosives. Phase stability and phase transformations are critical for applications, but despite many years of extensive research, the stabilizing mechanisms induced by additions, affecting important characteristics, such as mechanical properties, microstructure, and corrosion, are still not well understood.
Of particular interest for these applications is the face-centered cubic δ-phase, with unusual properties (negative thermal expansion coefficient, volume expansion, large low-temperature electronic specific heat, volume expansion), and the 5f electrons apparently in a mixed state, neither localized nor fully itinerant. The high-temperature phase is unstable in Pu, but becomes stable at ambient pressure and room temperature by the addition of trivalent elements.
Hydrogen is absorbed by plutonium exothermally, and hydriding does not require removal or disruption of the oxide layer. As long as hydrogen is available, the bulk material will hydride at a high rate1. It was shown that in metal-hydrogen (M-H) alloys, at high temperature and high hydrogen pressures, metal vacancies, called superabundant vacancies (SAV) are formed2,3. SAVs play a role in the stability of the M-H system, enhance the M-atom diffusion and creep, and have influence on mechanical, physical and chemical properties.
Hydrogen solubility studies of the Pu-H system showed that conditions for SAVs formation are favorable4. Helium release data from old Pu alloys is also consistent with the presence of H-induced vacancies. The issue is not completely understood yet, but a solution lies undoubtedly in considering the concomitant short- to medium-range structural distortions at the atomic level.
We present an experimental study of the local structure/short- and medium-range order around the plutonium and gallium atoms in δ-Pu-7at.%Ga alloys, hydrogen-loaded and unloaded, by total scattering technique that will provide a better understanding of the vacancy formation mechanism.
ACKNOWLEDGMENT
This work has been funded under 20110011DR and benefited from the use of the NPDF beamline at the Lujan Center at Los Alamos Neutron Science Center, funded by the US DOE, Office of Basic Energy Sciences. Los Alamos National Laboratory is operated by Los Alamos National Security LLC under DOE contract No. DE-AC52-06NA25396.

REFERENCES
1 J. Ward and JM Hashke Handbook on the Physics and Chemistry of Rare Earth vol 18
2 Y. Fukai J. Alloys and Compounds 356-357 (2003) 263-269
3 Y. Fukai and N. Nobuyuki, Phys. Rev. Letters, 37 (1994), 12, 1640
4 S. Richmond et al, 2010 IOP Conf. Ser.: Mat. Sci. Eng. 9 (2010) 012036

Pu6Fe is an intermetallic compound frequently found in small amounts in Pu alloys that contain iron impurities. The thermodynamic properties of this compound have not been studied extensively due to the difficulty of synthesizing this compound in single-phase form. The primary problem is the presence of the high temperature Pu2Fe phase which forms above ~428°C and tenaciously remains in the solid even after long anneals. We attempted to circumvent this problem by reacting Pu- and Fe-hydride powders to explore the possibility of forming Pu6Fe directly as the hydrides decompose during heating. Fe and Pu hydrides were prepared in the form of very fine powders, so that they could be intimately mixed, making the required diffusion distances quite small. The reaction product was examined using differential scanning calorimetry (DSC). The product was found to be predominantly Pu6Fe, mixed with a small amount of Pu. The enthalpy of melting for Pu6Fe was measured using DSC, and the values determined will be presented. An unusual 2-peak heat absorption structure was observed during melting that will be described in detail. The Pu-Fe phase diagram will be discussed in the 14 at. % Fe region in light of our results.

Understanding the interaction of impurities that may be present within plutonium (Pu) crystals is crucial from an applications standpoint, since such impurities may affect the structural and electronic properties of the material. It is well established that gallium (Ga) stabilizes δ-Pu from a high temperature phase to room temperature, and therefore it is the optimal phase for applications since δ-Pu is known to be ductile and malleable. Also the presence of Ga in δ-Pu makes it more corrosion-resistant in comparison to unalloyed Pu. Ab initio modeling allows us to gain atomistic insights into the processes that drive the interaction of the Ga impurities within the δ-Pu lattice. Using density functional theory (DFT), specifically the full-potential linearized augmented plane-wave DFT-based method, we present a systematic study of bulk Pu-Ga alloys and the interactions of atomic adsorbents with the (111) surfaces of the alloys. The goals of these calculations are to understand the atomic scale interaction of Ga in a δ-Pu lattice, along with understanding the precursors to Pu-Ga alloy surface corrosion. Bulk Pu-Ga alloys at 3.125 at. % Ga, 6.25 at. % Ga, and 9.375 at. % Ga concentrations were studied with a 32 atom δ-Pu supercell. Our results indicate that the equilibrium lattice constants decrease with increasing Ga concentration, in agreement with experimental observations. Furthermore, when more than one Ga impurity is present, the Ga atoms prefer to be at third nearest neighbor distance. Adsorptions of atomic hydrogen and oxygen on (111) surfaces of the alloys were then performed with a four-layer slab with eight atoms per layer. In general, the adsorbates bind strongly to the surfaces (~3 eV for H adsorption; ~8 eV for O adsorption). The changes induced on the surfaces by the adsorbates will also be discussed.
This work has been supported by the US Department of Energy through the Los Alamos National Laboratory LDRD Program.

The recent discovery of the first examples of complexes of +2 ions of yttrium, praseodymium, gadolinium, terbium, holmium, erbium, lutetium, and uranium by reduction of tris(cyclopentadienyl) complexes will be discussed along with the implications of the characterization data obtained on these species. Comparisons with analogous complexes of "traditional" +2 lanthanide ions will also be made.

Developing insight into how metal oxide electronic structure changes for a range compounds and materials will greatly benefit a variety of existing and emerging energy technologies. Many of the technologically desirable chemical, magnetic, electronic, and thermal properties of metal oxides are derived from strongly covalent metal-oxygen multiple bonds (metal oxos). Among approaches explored previously, ligand K-edge X-ray absorption spectroscopy (XAS) has emerged as an effective method for quantitatively probing electronic structure and orbital mixing. The presence of covalent mixing is observed as a pre-edge feature in the ligand K-edge XAS, which only has transition intensity if the final state metal orbital contains a component of ligand p orbital character. Recent advances have shown that insights regarding the nature of orbital mixing in metal oxides can be obtained at the K-edge for oxygen through a combination of XAS with a scanning transmission X-ray microscope (STXM), non-resonant inelastic X-ray scattering (NIXS), and hybrid density functional theory calculations (DFT).
Herein, a new effort is discussed that employs these techniques to understand bonding interactions in d- and f-block oxides. Oxygen K-edge XAS measurements and DFT studies began with a series of six tetrahedral oxyanions, MO₄^2- and MO₄^1- (M = Cr, Mo W and Mn, Tc, Re). Despite the similarities of the isoelectronic d0 MO₄^2- and MO₄^1- anions, unexpected differences in metal oxo orbital mixing were observed for adjacent metals in the periodic table. The lanthanide dioxides and sesquioxides, LnO₂ and Ln₂O₃ (Ln = Ce, Pr, Tb), were chosen for subsequent work because their electronic structures are well-established from hard X-ray spectroscopies. Features in the O K-edge XAS follow anticipated trends based on 4f and 5d orbital energies and occupancies. Overall, the research shows that orbital composition is influenced by a complex interplay between periodic changes in both orbital energy and radial extension.

Covalency is a fundamental concept in chemistry used to describe chemical bonding in s-, p-, d-, and f-block elements. However, given the restrictions of existing experimental techniques, the degree of covalency of a given bond is difficult to measure and is often estimated or inferred. This situation was recently altered by the pioneering work of Solomon, Hedman, and Hodgson who used ligand K-edge X-ray Absorption Spectroscopy (XAS) to directly measure covalency in bonding. We are currently expanding this technique to complexes that contain heavy atoms in an effort to improve contemporary descriptions of covalency, electronic structure, and bonding in actinides. These studies are providing unique insight to evaluate the relative roles of d- and f-orbitals in bonding, and these results will be presented in the context of recent advances in f-element chemical reactivity that cannot be easily explained using a traditional description of f-element electronic structure.

The octahedral U(V) complexes recently reported by Hayton and co-workers represent a unique opportunity to directly probe the strengths of pi and sigma interactions between uranium and a variety of ligands. These octahedral complexes, [UX6]-, consist of pentavalent uranium coordinated by a variety of ligands including alkyl (X = CH2SiMe3), alkoxide (X = OtBu), amide (X = NC5H10), and ketimide (X = N=CtBuPh). This family of complexes is ideal for studying f-orbital bonding it spans a range of commonly used ligands in organouranium chemistry and the spectroscopic data necessary to investigate the bonding is available. The bonding in these compounds may be compared to that in the well-known halide complexes ([UX6]-, X = F, Cl, Br; CeCl63-; PaCl62-; NpF6), which have been extensively investigated. In this study, the MO model developed by Thornley was modified to include the effects of covalency on spin-orbit coupling itself in addition to the effect of covalency on orbital angular momentum. This model is used to fit the optical and EPR spectra for the octahedral f1 complexes to determine the splitting of the f-orbitals. This information is then used to estimate the stabilization of ligand orbitals due to f-orbital bonding using the estimated energies of the metal and ligand orbitals. The results show that the oxidation state of the metal center is much more important than the identity of the ligand in determining the degree of covalency and the strength of the covalent bond formed between the ligand and the f-orbital. The results can be explained using a second order model.