The extraction of trivalent actinides from complex mixtures of dissolved spent fuel is of vital importance for potential recycling efforts and current remediation and storage strategies of high level waste. While much effort has been dedicated toward empirical strategies for optimizing extraction efficiency under a variety of conditions, the underlying fundamental physical mechanisms of extraction are often uncertain and the role of different physical forces upon extraction thermodynamics and kinetics are understudied. Toward this end, we present the results of multiple endeavors that include an quantum mechanical treatment of aqueous solvation of Np(III) as well as the thermodynamic favorability of extraction of Np(III) complexes of acetylacetonate, a model extracting ligand. The studies have been coupled to complementary molecular dynamics simulations that are beginning to reveal the complex but tractable physical phenomena that influence extraction efficiency.

The evolution of inert gas bubbles in metals has important implications on the evolution of the mechanical properties of nuclear materials as well as materials in highly irradiating environments, such as those expected in next-generation nuclear reactors. The presence of gas bubbles in metallic lattices can profoundly alter the mechanical properties and strength of materials leading to embrittlement, swelling, and blistering. The behaviors of gas bubbles are thus important components of any evaluation of the effects of irradiation-induced aging in a material. The alpha decay of plutonium in Puâ?"Ga alloys continually generates inert He atoms within the lattice of the Puâ?"Ga matrix. In naturally aged Pu specimens, those He atoms form into bubbles, He-filled vacancy clusters, with a characteristic size from 2-10nm. Upon annealing, the He bubbles are subject to temperature induced changes which results in a coarsening of the bubble distribution yielding a lower bubble density but larger average bubble sizes up to 60nm. The formation of the He bubbles in Puâ?"Ga alloys has been studied by TEM however concern has been raised that the preparation of very thin samples (> 1 micron) needed in the TEM experiment together with low number of voids in any particular TEM images may skew the measured He bubble concentration and distribution. To resolve these outstanding issues we have used a combination SAXS and USAXS to examine the formation and growth of He bubbles in aged and temperature annealed Puâ?"Ga alloys. Development of non destructive volumetric probes for nuclear materials is needed to confirm TEM results and validate models of Pu aging.

The complex redox chemistry of technetium-99 (⁹⁹Tc), a byproduct of Uranium-235 fission poses problems in closing the nuclear fuel cycle, remediation of waste tanks and immobiliztion of ⁹⁹Tc found in the environment. A strategy for the reduction of ⁹⁹TcO₄^-, pertechnetate and complexation of the low valent ⁹⁹Tc has been identified. Irradiation of Polyoxometalates (POMs), nanometer-sized metal oxides, with UV or white light, in the presence of a sacrificial electron donor, produces the reduced POM (â?oheteropolyblueâ?) that can in turn reduce ⁹⁹Tc^VIIO₄^-, to a low valent Tc species that can be stabilized by interaction with the reoxidized POM. In a specific case study, the α2-P₂W₁₇O₆₁^10- can be reduced, transfer electrons to reduce ⁹⁹Tc^VIIO₄^- to Tc^IV that is partially condensed into the POM structure as revealed by X-ray Absorption Spectroscopy studies. Electrochemical studies show that the Tc^IV oxidation state is accessible and reversible in this system under these conditions. Over time, the Tc^IV oxidizes and incorporates as Tc^V=O into the α2-P₂W₁₇O₆₁^10- framework. There are salient differences between Re and ⁹⁹Tc: reduction of Re^VIIO₄^-, results in Re^V immediately incorporated into the POM defect. The â?ointermediateâ? species is not observed for Re. Plenary Keggin ions can also reduce ⁹⁹Tc^VIIO₄^- to a low valent Tc species and stabilize the low valent Tc. In addition to serving as models for metal oxides, POMs may also provide a suitable platform to study the molecular level dynamics and mechanisms of the reduction and incorporation of Tc into a material.

Cation-cation interactions (CCIs), in which the oxo ligand from one neptunyl(V) ion bonds as an equatorial ligand to a neighboring neptunyl(V) ion, have been widely observed in solid-state compounds, where they influence structural motifs as well as physical properties. Actinyl compounds containing tetrahedral oxo-anions of the hexavalent group VI elements (S, Se, Cr, and Mo) have been relatively well studied due to their environmental and technological importance. To further explore the role of lattice dimensionality, as promoted through CCIs, on the magnetic and vibrational properties exhibited by neptunyl(V) compounds, we have synthesized and characterized numerous neptunyl(V) sulfates and selenates. More than half of them contain a CCI network, where each NpO2+ cation participates in two, three, or four CCIs. The CCI lattices range from one-dimensional NpO2+ chains, ribbons, two-dimensional sheets to condensed three-dimensional structures. Most of the selenates are isostructural with their sulfate analogues, although the structures of two compounds slightly differ from those of sulfates. The divergence in the NpO2+-sulfate and -selenate interactions is consistent with the high energy X-ray scattering (HEXS) studies on UO22+-sulfate and selenate ion pairs in aqueous solutions. Magnetic measurements conducted on several compounds, all with square arrangements of neptunyl(V) cations through CCIs, show a ferromagnetic ordering. IR and Raman spectra of compounds possessing CCIs are more complex than those without CCIs at the regions attributed to O=Np=O vibrations. Generally consistent with expectation, the stretching band of neptunyl units involving CCIs shift significantly towards lower frequencies. Details will be presented and discussed. This work is supported by the U.S. DOE, OBES, under contract DE-AC02-06CH11357. References: 1. Jin, G. B.; Skanthakumar, S.; Soderholm, L. Inorg. Chem. 2011, 50, 6297-6303. 2. Jin, G. B.; Skanthakumar, S.; Soderholm, L. Inorg. Chem. 2011, 50, 5203-5214.

Heavy fermion 4f superconducting materials all appear to be found in the vicinity of a magnetic quantum critical point. Given the strong similarities in properties of 4f and 5f heavy fermion materials, one naturally wonders if this presence of quantum criticality with superconductivity also is the case for the 5f heavy fermion superconductors. We discuss 5f heavy fermion superconductivity from thts viewpoint.

Intermetallic compounds containing actinide ions exhibit braod spectrum of different physical phenomena at low temperatures. The latter include heavy quasiparticles, unconventional superconductivity and various forms of magnetic ordering. The complex and sometimes enigmatic properties of these compounds derive from the strong correlations among the 5f electrons. Previous model calculations suggested that the intra-atomic Hund's rule-type correlations may lead to partial localization which is reflected e. g. in the co-existence of itinerant 5f-derived heavy quasiparticles and local magnetic excitations. The conjectured "dual nature" of 5f electrons which is closely related to the question of the 5f valence of the actinide ions is not directly probed by ground state properties and the low-energy excitations. Here we present microscopic calculations for electron spectroscopies emphasizing the consequences of strong intra-atomic correlations of the 5f shell. Starting from the Anderson model in the mixed-valence regime results for a single impurity embedded in a metal and for small clusters are discussed.

Solution aldol condensation of two acetone (ACO) molecules to diacetone alcohol (DAA), followed by dehydration to mestiyl oxide (MO), is represented by the following sequence:
       â?oACOâ?Â                          â?oDAAâ?Â                              â?oMOâ?
2 CH₃C(O)CH₃ â?" CH₃C(O)CH₂C(OH)(CH₃)₂ â?' CH₃C(O)CH=C(CH₃)₂ + H₂O The alternative pathway of retro-aldol dissociation of DAA to ACO is indicated by the double arrow in the above reaction.

Remarkably, gas-phase aldol chemistry in uranyl complexes with the bidentate DAA ligand can be induced by excitation, which provides energy to surmount kinetic and/or thermodynamic barriers to ligand rearrangement. In collision-induced excitation (CIE), charged complexes are isolated in an ion trap and undergo energetic collisions with helium to produce an excited state intermediate in which dehydration or retro-aldol chemistry can occur, as in the following examples:
[UO₂(DAA)₂]²⁺ â?' {[UO₂(DAA)₂]²⁺}* â?' [UO₂(DAA)(MO)]²⁺ + H₂O [UO₂(DAA)₃]²⁺ â?' {[UO₂(DAA)₃]²⁺}* â?' [UO₂(DAA)₂(ACO)]²⁺ + ACO
As these examples illustrate, dehydration is prevalent for low-coordination complexes, and retro-aldol dissociation is prevalent for high-coordination complexes.

A new alternative to CIE is ligand-addition excitation (LAE), whereby exothermic addition of a Lewis base to a complex provides excitation energy, as in the following examples where L = tetrahydrofuran (THF):
[UO₂(DAA)₂]²⁺ + L â?' {[UO₂(DAA)₂(L)]²⁺}* â?' [UO₂(DAA)(ACO)(L)]²⁺ + ACO [UO₂(DAA)(ACO)₂]²⁺ + L â?' {[UO₂(DAA)(ACO)₂(L)]²⁺}* â?' [UO₂(MO)(ACO)₂(L)]²⁺ + H₂O
As seen in the example of [UO₂(DAA)₂]²⁺, the products of CIE and LAE are not necessarily the same. For high-coordination complexes, addition of a strong base results not in LAE but rather in displacement of a weaker base ligand, as in the following example of ligand-exchange:
[UO₂(DAA)₂(ACO)]²⁺ + THF â?' [UO₂(DAA)₂(THF)]²⁺ + ACO

The means by which intriguing new aldol chemistry in isolated gas-phase uranyl complexes has been unraveled will be described. Results for â?oconventionalâ? CIE of gas-phase uranyl DAA complexes will be compared with those for the new approach of LAE, which in some regards resembles photo-excitation. Aldol chemistry is also reported for gas-phase plutonyl complexes.

This work was supported by the Office of Basic Energy Sciences, U.S. Department of Energy.

The U-TRU-Zr and U-TRU-Mo alloys proved to be very promising fuels for TRU-burning liquid metal fast breeder reactors. The optimal composition of these alloys is determined from the condition that the fuel could remain stable in the bcc phase (γ-U) in the temperature range of stability of α-U phase. In other words, both Zr and Mo play a role of â?~γ-stabilizersâ?T helping to keep U in the metastable bcc phase upon cooling. The main advantage of U-Pu-Mo fuels over U-Pu-Zr fuels lies in much lower constituent redistribution due to the existence of a single γ-phase with bcc structure over typical fuel operation temperatures. The nucleation time for the decomposition of the metastable alloys, which controls the constituent redistribution process, is directly connected with the excess enthalpy of solution of these alloys. In the present study we perform KKR-ASA-CPA and EMTO-CPA calculations of the ground state properties of γ-U-Zr and γ-U-Mo alloys and compare their heats of formation with CALPHAD assessments. We discuss how the heat of formation in both alloys correlates with the charge transfer between the alloy components, and how the specific behavior of the density of states in the vicinity of the Fermi level promotes the stabilization of the U₂Mo compound. Our calculations prove that, due to the existence of a single γ-phase over the typical fuel operation temperatures, γ-U-Mo alloys should indeed have much lower constituent redistribution than γ-U-Zr alloys where a high degree of constituent redistribution takes place. The binodal decomposition curves for γ-based U-Zr and U-Mo solid solutions are derived from Ising-type Monte Carlo simulations incorporating effective cluster interactions. We also explore the idea of stabilization of the δ-UZr₂ compound against the α-Zr (hcp) structure due to increase of Zr d-band occupancy by the addition of U to Zr. Analogy with stabilization of the Ï?-phase in Zr under compression is made. Though the U-Pu-Zr and U-Pu-Mo alloys can be used as nuclear fuels, a fast rector operation on a closed fuel cycle will, due to the nuclear reactions, contain significant amount of MA (Np, Am, and Cm). Calculated heats of formation of bcc Pu-U, Pu-Np, Pu-Am, Pu-Cm, Pu-Zr, Pu-Mo, Np-Zr, Np-Mo, U-Am, Np-Am, and U-Cm, alloys are also presented and compared with CALPHAD assessments. This work was performed under the auspices of the US Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

Plutonium uranium mixed oxide (MOX) fuels are currently used in nuclear reactors. The actinides in these fuels need to be analyzed after irradiation for assessing their behaviour with regard to their environment and water (coolant, reactor pool, repository groundwater). In this work the study of the atomic structure and next-neighbour environment of Am in the (Pu,U)O2 lattice in an irradiated (60 MW.d/kg) MOX sample was performed employing micro-X-ray fluorescence (µ-XRF) and micro-X-ray absorption fine structure (µXAFS) spectroscopy. The chemical bonds, valences and stoechiometry of Am (~0.66 wt%) are determined from the experimental data gained for the irradiated fuel material examined in its peripheral zone (rim) of the fuel. In the irradiated sample Am builds up as Am3+ species within an (AmO8)13- coordination environment (>90%) and no (<10%) Am(IV) or (V) can be detected in the rim zone. The occurrence of tetravalent americium is likely to be avoided by the buffering activity of the uranium dioxide matrix. This conclusion is justified by thermodynamic modelling the system UO2 + AmO2 <=> UO2+x + Am2O3 either as pure solid phases, or as ideal or non-ideal MOX solid solutions using the GEM-Selektor code.

A detailed knowledge on the temperature rise associated with plutonium hydriding will improve the safe handling of plutonium and protection of the environment. Unfortunately, the heat generated and the temperature profile during hydriding at the local level has never been measured. In this paper, the utilization of an infrared pyrometer camera to map the spatial and temperature profiles of plutonium hydriding is presented. Experiments were carried out with plutonium coupons held in vertical position and at a 45o inclined angle. A great difference in the temperature profiles of the Pu coupon surfaces for the different mounting positions is observed and can be attributed to the spallation and landing of hydriding debris/powder. The hydride debris/powder started out as thermally hot PuH2+x with x < 1 and continued to react with H2 to form PuH3, releasing more heat. However, in the vertically mounted position, most of the hot and reacting PuH2+x debris/powder fell off the coupon under the force of gravity, carrying with it much of the heat. So, the temperature rise associated with hydriding corrosion varies according to the orientation of the Pu coupon. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory in part under Contract W-7405-Eng-48

Plutonium is known to interact strongly with hydrogen, particularly at elevated temperatures. We have measured the hydrogen uptake of Pu-2 at. % Ga using a Sievertâ?Ts apparatus and found the terminal solubility of hydrogen in this alloy to be between 1 and 2 at. %, above which the PuH2 phase begins to form. We are particularly interested in the pre-hydriding solution region, and believe that there is evidence for a hydrogen-induced vacancy multiplication mechanism in this region. We have performed hydrogen absorption experiments in a Sievertâ?Ts apparatus to determine the solubility constant (Ks) at a variety of temperatures and hydrogen concentrations. Our work demonstrates a steep increase in Ks with increasing hydrogen concentration, in the dilute regime below 0.1 at. % hydrogen. We believe this is due to a vacancy multiplication effect wherein hydrogen atoms in the lattice induce vacancies or vacancy complexes. These in turn act as attractive sites for additional hydrogen atoms, thereby increasing the solubility of hydrogen. In concert with this experimental work, we are also calculating the energy of various hydrogen-vacancy configurations using density functional theory. We observe a monotonically decreasing energy for hydrogen-vacancy complexes as the number of vacancies is increased, which we feel supports the proposed vacancy multiplication mechanism. The results of this ongoing investigation will be discussed in detail, as well as future directions.

Synchrotron-based X-ay techniques are used increasingly to characterize actinide element speciation in heterogeneous media related to nuclear waste disposal safety. Especially techniques offering added temporal, spatial and energy resolved information are advancing our understanding of f-element physics and chemistry in general and of actinide element waste disposal in particular. Examples of investigations of actinide and lanthanide containing systems using both highly (energy) resolved X-ray emission spectroscopy (HRXES) techniques and spatially resolved techniques with focused X-ray beams will be presented. These include HRXES results as polarization dependent partial fluorescence yield X-ray absorption near edge structure (PD-PFY-XANES) spectroscopic studies of single U(VI) crystals, which reveal a splitting of 6d valence states experimentally for the first time [1], and an example of using an innovative combination of techniques (micro- and nano-X-ray fluorescence confocal mapping & tomography, XANES, and in-line X-ray phase contrast tomography) for investigation of Np immobilization in granite following a radiotracer experiment [2,3]. At the end of the talk, an outlook of new experimental facilities such as PETRAIII for future studies of actinide systems will be given. REFERENCES 1) T. Vitova, M.A. Denecke, â?oHigh-resolution polarisation dependent x-ray absorption fine structure (HR-P-XAFS) study of uranyl complexesâ?, European Synchrotron Radiation Facility Report (2010). http://ftp.esrf.eu/pub/UserReports/41191_A.pdf 2) Melissa A. Denecke, Wout de Nolf, Alexander Rack, Remi Tucoulou, Tonya Vitova, Gerald Falkenberg, Sousan Abolhassani, Peter Cloetens, Bernhard Kienzler, Speciation of actinides in granite subjected to tracer studies, in Actinide Nanoparticle Research (1st Edition), Kalmykov, S. N.; Denecke, M. A. (Eds.) Springer Verlag, Heidelberg, 2011. 3) M.A. Denecke, B. Brendebach, W. De Nolf, G. Falkenberg, K. Janssens, R. Simon, Spatially resolved µ-XRF and µ-XAFS study of a fractured granite bore core following a radiotracer experiment Spectrochimica Acta B 64, 791-795(2009). doi: 10.1016/j.sab.2009.05.025

Nuclear weapon tests in the 60s and industrial accidents released non-negligible quantities of radionuclides (RNs) in the environment. This radioactive pollution has been studied by our laboratory since the 80s in south-east of France and particularly in the â?oParc National du Mercantourâ? . The aim of this new investigation is to better understand the anthropogenic dispersion of contaminating RNs in natural environment. For this, a protected catchment basin located in a wooded area at 1742 m in the Boreon mountain massif has been selected. Natural samples (soil, sediment, water) were collected in a specific mountain artificial water damming drainage basin and treated in our laboratory. Both anthropogenic and natural RNs have been measured after chemical separation and concentration in the laboratory. Due to the difficulty to model the reactivity of the RNs in natural samples because of the intrinsic intricacy of their chemical physical properties, a second step in our work lead us to focus on simplified chemical systems using chemical analogues. Natural (238U, 232Th) and anthropogenic (137Cs, 238,239,240Pu, 241Am) RNs and trace elements (Li, As, Cs, Nd, Eu, Lu, Th, Uâ?¦) were quantified using mass and nuclear spectrometries. A few concentration profiles have been significantly perturbed by a dredging of the water damming in 1991-92. Tested trace elements and natural U profiles suggest that the sediment cores have two different behaviors: a â?olake sedimentâ? type and a â?osoilâ? type. Interestingly, Pu has a paradoxical behavior. Although Pu appears to be more mobile than Cs in the depth profiles, the ratios between leached Pu and Cs are stable. Given the lack of bibliographic data on Am, comparison of these data with Am profiles in soil and sediments has also been undertaken. In order to go beyond this macroscopic description, the chemical reactivity of the RNs versus soil and sediment must be explored. However the complexity in terms of geochemical characterization of the natural medium lead us to restrain our methodology to simple models. Since 241Am has no complicated RedOx chemistry, it is one of the best candidate for this methodology. Indeed, lanthanide analogues like Nd may be used to simulate the Am behavior in simplified model chemical systems . The combination of those data allows us to better understand the RN behaviors in natural compartments. In the future, inventory results related to sorption data on model systems may be integrated in a geochemical model in order to predict the behavior of RNs in case of a new accidental dispersion. 1 Barci, G.; Dalmasso, J.; Ardisson G. J. Radioanal. Nucl. Chem. 1987, 117, 337-46 2 Rezzoug, S.; Michel, H.; Fernex, F.; Barci-Funel, G.; Barci, V. J. Environ. Radioact. 2006, 85, 369-79 3 Silva, R.J.; Bidoglio, G.; Rand, M.H.; Robouch, P.B.; Wanner, H.; Puigdomenech, I. Chemical Thermodynamics of Americium, 1995, Chemical Thermodynamics Series Volume 2

The demands of nuclear fuel modeling place metal oxides in contact with metals and alloys at both the fuel-cladding interface and within oxide-dispersion strengthened steels. The intimate contact necessitates that atomistic models for these materials be compatible and consistent with each other at some level. Presently, most atomistic models address only metals, or only ceramics, but rarely both. In addition, the cations in the actinide oxides themselves enter multiple oxidation states, depending on the composition. Actinide metals themselves present challenges to atomistic models because of strong electron correlation effects. Several recent efforts to meet these challenges are presented here. One effort addresses phase ordering in Pu metal. In that effort, the correct phase ordering between fcc and hcp phases are enforced, and the effects of that enforcement on other material properties such as dislocation core structures, surface relaxation, and grain boundaries are investigated. In addition, the nature of simple point defects is compared to what is found in existing models. The revised model for Pu metal comes from allowing a competition between different components of the valence electron density, of a particular symmetry type, as a function of volume. A second effort investigates a new, "fragment'' Hamiltonian (FH) model, at the atomistic level, that can represent different cation charge states and reduces qualitatively to exiting, successful models for metals, such as the embedded atom method. Moreover, the FH model possesses both electron hopping and fundamental gaps that appear as separate terms in a generalized embedding function. The electron hopping contributions come from both one-electron and two-electron sources in the model. The model obeys certain well-known theoretical limits that come from the nonlinearity of electron hopping processes as the volume of a crystal is changed. The generalized notion of embedding entails two variables instead of one. The ability to account for multiple charge states in the cations leads to the capability within the model to distinguish the qualitative differences among metallic, ionic, and covalent bonding environments. The FH model is applied to the structure of representative metal - oxide interfaces. The relative character of the materials as far as their metallic and ionic qualities are also assessed.

Plutonium oxide heat sources are used to power space missions. The 238 isotope of Pu generates heat through alpha decay. The heat is converted to electricity in a thermopile, providing electricity for years. Pu-238 has an 88 year half-life. Decay of the Pu produces helium and uranium, and consequently most sources are vented to allow release of He from the case. A small number of small, low-power sealed sources exist, and examination of these sealed sources provides an interesting window into aging behavior of the oxide, and capture of evolved helium in the oxide matrix. All of the helium produced in decay can be contained in the oxide lattice, where it occupies the tetrahedral sites. Some helium diffuses out at a rate last measured at oxide ages of about 1 year, with a rate that is somewhat dependent on the form and morphology of the fuel. Current measurements on sealed sources as old as 34 years indicate that the rate of diffusion has changed only slightly over time. Mechanisms for helium release include bubble diffusion, point defect migration, and agglomeration and movement of He at grain boundaries. We observe primarily point defect diffusion, despite the rising concentration of helium in the lattice over time. Because of the slow diffusion of helium from the fuel to the headspace, sealed sources anticipated to be stable and useful over a long lifetime.

The phonon spectrum of alpha-uranium has been measured by Manley {\it et al.} in a series of experiments using inelastic neutron and x-ray scattering. The 2001 results showed that the optic modes soften by a few meV as the temperature is increased between room temperature and 450 K. In 2006, a new dynamical mode was observed to form above 450K which the authors attribute to an intrinsically localized mode, which stabilized by anharmonic interactions. We propose a possible alternate cause for the formation of the new mode and the softening which is based on the existence of strong electron-phonon interaction together with a low excitation energy for transitions between states with f and conduction electron characters. The model allows for a resonant interaction between the optic phonons and the electronic excitations, which may lead to the high-energy peaks in the phonon spectra splitting and acquiring mixed electronic and phonon characters.

Using inelastic x-ray scattering, we determine the phonon density of states of PuO₂(+2%Ga) and compare results with recent predictions made using Density Functional Theory (DFT), DFT plus the Hubbard U (DFT+U), and Dynamical Mean-Field Theory (DMFT). The DFT prediction underestimates the measured energies of most features. The DFT+U and DMFT predictions more accurately reflect the low energy features but fail to capture the highest energy features. Ramifications for predictions made of thermodynamic and transport properties of this important nuclear fuel material will be discussed.

Cast pucks and rods of high density alpha plutonium were created as a part of R&D efforts at Los Alamos National Laboratory. The as-cast material was analyzed using a variety of characterization techniques, including immersion density, resonant ultrasound spectroscopy (RUS), dilatometry and quasi-static mechanical testing. 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. Stress-strain curves from quasi-static compression tests will be presented as well. This data will be compared to previous literature values and similar experiments done on aged alpha material.

The actinides and their compounds exhibit unique structural properties. In particular, the 5f-electrons in these materials change from itinerant to confined behavior over a relatively narrow range of temperature, pressure, and chemical environment. Electronic structure calculations based on density functional theory (DFT) using available approximations to the exchange/correlation functional fail to capture this behavior. In addition, DFT calculations for the light actinides (Th-Np), where the 5f-electrons are thought to be itinerant and bonding, agree poorly with experiment compared with calculations of lighter elements using the same functionals. Accordingly, the functional that works best for the actinides is not as satisfactory for most other materials. In order to develop a predictive capability for these materials, it is important to distinguish between failures in the underlying physics from errors arising from an inadequate representation of that physics. The precise treatment afforded by an accurate first-principles calculation based on the full Dirac equation allows definitive predictions of material properties and provides a stringent test of new density functionals designed to accurately capture the behavior of heavy relativistic materials. In this talk we describe the use of electronic structure calculations based on the full Dirac equation to predict the structural properties of selected lanthanide and actinide elements. It has been demonstrated [1] that even the most basic property of these materials -- the equilibrium volume -- is impossible to predict with certainty without using the Dirac equation to produce semi-core p-bands. In this work we demonstrate the effect of full relativity on predictions of material properties and compare with previous calculations obtained using less accurate approaches. Sandia National Laboratories is a multi-program laboratory managed andoperated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. Work at LANL was supported by the U.S. Department of energy under Contract No. DE-AC52-06NA25396. [1] L Nordstrom et al, Phys. Rev. B 63 2001, 35103..

Electrical resistivity of PuAl₂, first reported by Arko et al in 1972, increases from 150 μΩcm at 300 K up to 220 μΩcm at ~10 K and then drops precipitously to residual resistivity at ~3 K. This drop implies an electron scattering mechanism is rapidly freezing out as temperature T approaches zero. Since no magnetic ordering is reported in PuAl₂, this phenomenon is apparently unrelated to static magnetism. Low-T resistivity increases from residual resistivity as T2, characteristic of dynamical spin fluctuations (SFs). However, properties of PuAl₂ might be more complex than indicated by early work and additional measurements might provide more insight into the nature of this material. We have measured magnetic susceptibility Ï?, electrical resistivity ρ, heat capacity CP, and thermoelectric power S of small 239PuAl₂ single crystals from 300 K to 2 K with finer temperature resolution than previously. All new measurements show additional anomalies in low-T properties. Ï? is virtually coincident with Arko down to 50 K and has a small slope change at 35-40 K. Below 40 K our Ï? values are somewhat larger but indicate a slope change at ~10 K and a peak at 3-4 K. ρ has a slight slope change at 40 K, a broad peak of 220 μΩcm between 6-9 K, followed by a plunge to 100 μΩcm at 2 K. Our CP data have peaks at 8 and 3 K, close to the corresponding ones of Stewart and Elliot. However, our CP values and peak widths are substantially larger than theirs. Our CP data have a small plateau at 37 K. S is +10 μV/K at 300 K and decreases through zero at 120 K. A small slope change occurs at 35 K, S reaches a minimum of -12 μV/K at 20 K and then increases and goes through small maximum at - (3 to 4) μV/K and 3-4 K. All these data have a small slope change at 35 to 40 K. Radiation damage lowers monatonically our values of Ï? and CP. At 300 K the cubic crystal structure of PuAl₂ remains unchanged up to 38 GPa in a diamond anvil cell. Our new measurements on 239PuAl₂ single crystals show additional anomalies, which indicate PuAl₂ is more complex than thought previously. SFs remain the likely mechanism for the low-T behavior. PuAl₂ remains a paradigm of SFs. However, the presence and intrinsic nature of SFs in PuAl₂ need to be de demonstrated with a diagnostic capable of detecting variation of spin-fluctuation frequency with decreasing temperature below ~20 K. A virtually perfect crystal is needed to find the intrinsic nature near T=0 K. Thus, self-radiation damage induced by 239Pu must be eliminated by use of 242Pu and the problem of obtaining perfect order in a line compound must be solved by metallurgical research.

The electronic structure of many of the oxides containing d and f-elements has long been a challenge for theory. For example, the traditional workhorses of density functional theory(DFT), the local density approximation (LDA) and the generalized gradient approximations (GGA), predict most of these systems to be metallic, when in fact they are insulators with band gaps of several eV. These problems reflect the localization/delocalization dilemma faced in systems with weak overlap. In this talk I will review the results of a number of approximations designed to deal with these problems and compare their predictions with recent experimental band gaps measured on single-crystal quality epitaxial thin films, photoemission measurements of the occupied bands, and X-ray absorption studies of the unoccupied states. The many-body approaches considered here include the self-interaction correction (SIC), the GGA+U approaches, dynamic mean-field theory(DMFT), and hybrid DFT.

The electronic structure and high-pressure phase transitions of stoichiometric UO₂ and hyperstoichiometric UO_2.03 were investigated using first-principles calculations. The generalized gradient approximation and projector-augmented wave method with on-site Coulomb repulsive interactions were applied in order to calculate the equilibrium volume, total and partial density of states, band gap, and energetics of the high-pressure phase transitions. With optimized Hubbard U parameter (U=3.8, J=0.4) based on experimental band gap width, the calculated cell parameter for the stoichiometric UO₂ with cubic fluorite-structure is ~1 % greater than experimental value, as compared with ~1 % smaller without on-site Coulomb repulsive interactions. For hyperstoichiometric UO_2.03 with the cubic fluorite structure, the interstitial oxygen at the octahedral interstitial site induces new bands within the band gap of stoichiometric UO₂. In contrast, no new bands appear in the band gap for the orthorhombic cotunnite high-pressure phase. Charge transfer to the interstitial oxygen in the hyperstoichiometric UO_2.03 is partially delocalized in the cubic fluorite structure, but fully localized in the orthorhombic cotunnite structure. The energy required for the incorporation of an interstitial oxygen is 0.3 eV higher for the orthorhombic structure with respect to for the cubic structure at ambient pressure and increases to 0.5 eV at 10 GPa. The calculated transition pressures from the cubic to the orthorhombic structures are 17 and 27 GPa for UO₂ and UO_2.03, respectively. The dramatic increase in the transition pressure for the hyperstoichiometric uranium dioxide is related to the structural incompatibility of the interstitial oxygen in the cotunnite structure (high-pressure phase) with respect to the fluorite structure. These results suggest that experimentally determined pressures for the phase-transition are significantly affected by small compositional deviations off the ideal stoichiometry UO₂.

Besides the importance of AnO₂ series (An; U, Np, Pu, Am) as a nuclear fuel, the magnetic properties of these compounds at low temperatures are particularly interesting. Their surprisingly varied physical properties continue to be of interest for both theory and experiment. In this study, we have performed NMR studies for the series of actinide dioxides. On the basis of ¹⁷O-NMR studies, exotic magnetic orderings associated with multipole degrees of freedom on 5f electrons have been identified in UO₂ (dipolar + quadrupolar ordering) and NpO₂ (octupolar + quadrupolar ordering), in contrast with the non-magnetic ground state of PuO₂. In AmO₂, our ¹⁷O-NMR data provide the first microscopic evidence for a phase transition as a bulk property in this system. In the ordered state, we have observed a spectrum with a triangular lineshape, which resembles neither that of UO₂ nor NpO₂. In addition to the magnetic properties at low temperatures, NMR can reveal the defect state at high temperatures, which would be quite useful to characterize nuclear fuel.

All actinide materials melt from the body-centered cubic (bcc) crystal structure. This phase is particularly difficult to model because the first-principles density-functional-theory (DFT) approach predicts bcc to be mechanically unstable in the actinides. Therefore, using DFT zero-temperature results as a starting point for modeling the high-temperature bcc phase is problematic. Recently a new efficient technique was proposed to deal with this problem more generally. The self-consistent ab initio lattice dynamics (scaild) method was shown to sufficiently include entropy contributions to stabilize bcc Ti, Zr, and Hf at elevated temperatures even though these metals are unstable in the bcc phase at zero temperature [1]. Here we adopt this technique to study the actinide metal uranium from which we will present results. [1] P. Souvatzis, O. Eriksson, M. I. Katsnelson, and S. P. Rudin, Phys. Rev. Lett. 100, 095901 (2008). This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

Heavy fermion superconductor is characterized by its 'heavy' conduction electrons condensing in Cooper paris. Particularly NpPd5Al2 is one of the recently examples in such material. It shows superconductivity below 5 K with a large electronic specific heat in the normal state corresponding to the heavy fermion state. We developed a Faraday magnetometer suitable for radioactive material to investigate the magnetization characteristics at low temperature (0.1 K) and high field (15 T). Magnetization of NpPd5Al2 clearly shows discontinuous change at the upper critical field, demonstrating the Pauli-limited first order transition. The present results are discussed in comparison with other rare-earth and actinide based heavy fermion superconductors.

Transuranium (TU) intermetallics show a large variety of exotic behaviors mainly coming from the 5f-ligand hybridization. Physical phenomena like long-range magnetic order, heavy-fermion ground state, and/or "non-Fermi liquid" behavior helped raised interest in these systems. Despite fascinating intrinsic properties, it has always been difficult to proceed with deep analysis of transuranium compounds due to their radioactivity and radiotoxicity. The interest of the scientific community has been re-focused on the actinoid-based compounds when superconductivity in Pu-based ternaries, namely, PuCoGa₅+, PuRhGa₅ and PuCoIn₅ was reported at 18.6, 9 and 2.5 K, respectively. Recently, Aoki et al. have observed superconductivity in heavy fermion NpPd₅Al₂ with Tc = 5 K. This first Np superconductor was found to be a paramagnetic heavy fermion system with γ~300 mJ/mol.K2. The low-temperature specific heat (also NMR measurements) agrees with the presence of line nodes in the energy gap characteristic of strong coupling d-wave superconductivity. Electrical resistivity and heat capacity measurements under magnetic fields indicate the existence of a field-tuned quantum critical point around Hc2 and the recovery of a Fermi-liquid behavior above Hc2. Motivated by all these findings and searching for a new TU-based superconductors, we have synthesized and widely examined members of AnPd₅Al₂ (An = Th, U, Np, Pu Am) family. The compounds were prepared and characterized by means of powder and single crystal x-ray diffraction as well as by energy dispersive x-ray analysis (EDX). By performing extensive magnetic, transport, thermal and thermodynamic studies we try to connect some physical characteristics with the presence of the unconventional superconductivity in this family. We discuss the implications of these results. All experiments have been performed in a wide temperature (0.5-300 K) and magnetic field ranges (0-14 T).

Superconductivity is a rare phenomenon in transuranium compounds. Despite intensive work on neptunium and plutonium-based materials, only a few superconductors have been reported in the last decade. Without any doubt, the more fascinating material is PuCoGa₅ with T_c =18.5 K while PuRhGa₅ presents superconductivity at 8.5 K and PuCoIn₅ at 2.5 K [1,2,3]. All of them have the HoCoGa₅-type of crystal structure, similar to CeTIn₅ heavy fermion superconductors (T = Co, Ir at ambient pressure and Rh under pressure). Recently, a new family of superconductors, based on the ZrNi₂Al₅-type has been reported with NpPd₅Al₂ (T_c=4.9 K) and CePd₅Al₂ (T_c~0.5 K under pressure) [4,5]. The origin of superconductivity in both transuranium families is still unclear but spin fluctuations would be a serious candidate as has been proposed for the CeTIn₅ compounds. Here we report on synthesis processes, material study and basic property examinations of the plutonium-based compound related to this new structure, namely PuPd₅Al₂. Several synthesis and characterization routes have been performed (arc melting, Bridgman, powder and single crystal X-ray diffraction, EDX) for polycrystalline and single crystal fabrication. Also, a low self heating isotope (²⁴²Pu) has been used for synthesis to allow low temperature studies. We have then compared basic properties such as magnetism, transport and thermodynamic behavior of PuPd₅Al₂ with its cerium counterpart. Finally, since CePd₅Al₂ exhibits pressure induced superconductivity, we have performed a high pressure study on a PuPd₅Al₂ sample up to 6 GPa. [1] J. Sarrao et al., Nature (London) 420, (2002) 297. [2] F. Wastin et al., J. Phys.: Cond. Matter 28, (2003) 2279 [3] E.D. Bauer et al, to be published [4] D. Aoki, J. Phys. Soc. Jpn 76, (2007) 063701. [5] F. Honda, J. Phys. Soc. Jpn. 77, (2008) 043701.

One hundred years after its discovery, many fundamental questions about superconductivity are still open, particularly those concerning the microscopic mechanism that gives rise to the pairing of electrons in the so-called unconventional superconductors where the simple electron-phonon coupling seems not to be sufficient to explain the formation of the condensate. PuCoGa₅, which is a 5f-electron heavy-fermion superconductor, is one of such examples. It has a number of interesting and peculiar properties, including the highest critical temperature among heavy fermions (T_c = 18.5 K). To obtain such a high T_c, a standard electron-phonon mechanism would require a coupling constant, λ much higher than the experimentally determined one. A magnetic origin of the pairing seems instead to be likely for various reasons, but is difficult to reconcile with the temperature-independent susceptibility in the normal state. Unambiguously determining the symmetry of the superconducting order parameter (OP) is very helpful in this context, since it can contribute to clarifying the nature of the pairing mechanism. Here we report on point-contact spectroscopy results on PuCoGa₅ single crystals, which show for the first time in a direct and unambiguous way that the superconducting OP has d-wave symmetry. The obtained gap amplitude is Î"(0)=5.1 ± 0.3 meV, which corresponds to a gap ratio 2Î"/k_BT_c = 6.5 ± 0.3, indicating that the compound is in the strong-coupling regime. The same gap ratio has been obtained also on crystals with T_c = 14.5 K due to the presence of a small amount of impurities. In both kind of crystals the gap value and its temperature dependence can be well reproduced within the Eliashberg theory for superconductivity if the spectral function of the mediating bosons has a spin-fluctuations-like shape, with a peak at about 6.5 meV, compatible with the energy range of spin fluctuations recently determined by NMR measurements. This fact indicates that spin fluctuations are the probable boson that mediates superconductivity. This scenario must however be reconciled with the observed temperature-independent magnetic susceptibility of PuCoGa₅, that points to vanishing local moments at the Pu sites. To do so, we performed electronic structure calculations combining the local density approximation with an exact diagonalization of the Anderson impurity model, i.e. accounting for the full structure of the f-orbital atomic multiplets and their hybridization with the conduction bands. We found out that the Pu f shell fluctuates between a magnetic sextet and a non-magnetic singlet, thus carrying a non-vanishing average moment. However, these fluctuations are accompanied by canceling antiferromagnetic fluctuations in the conduction bands, which can be viewed as a manifestation of the Kondo physics. In other words, the time-dependent 5f local moment is dynamically compensated by a moment momentarily formed in the surrounding cloud of conduction electrons.

The discovery of superconductivity in the family PuMGa₅ (M=Co, Rh) of transuranium compounds has attracted significant research interest in recent years. The true character of the superconducting order parameter and, especially, the origin of the pairing glue for the superconducting condensate in these compounds are still under debate. Very recently, the first In analogue of the PuMGa₅ family was synthesized, PuCoIn₅, offering the ground for further investigation of these puzzles. In the first part of this talk, an overview of the physical properties of PuCoIn₅ and a summary of several thermodynamic measurements will be presented. Subsequently, our comprehensive Nuclear Quadrupole Resonance (NQR) study, for a wide range of temperature (T~ 0.25K-300K), will be discussed in detail. From the analysis of the NQR spectra the quadrupolar parameters are deduced in excellent agreement with calculated values, while a superconducting critical temperature of T_c = 2.2K is determined. Moreover, the temperature evolution of the spin-lattice relaxation rate (1/T₁) exhibits characteristic behavior in different regimes: Above T_k~100K, 1/T₁ is governed by a temperature independent process, associated with the local spin fluctuations of 5f electrons. A Korringa type slowing of the relaxation is observed below T_k and down to T~10K, where antiferromagnetic spin density fluctuations are established as manifested by the power law decay of 1/T₁ for T_câ?¤Tâ?¤10K. Lastly, below T_c = 2.2K, the temperature dependence of the relaxation rate is well described by a nodal anisotropic superconducting order parameter.

Electron energyâ?"loss spectroscopy (EELS) in the transmission electron microscope (TEM) enables the chemical and electronic investigation of a material at a very high spatial resolutions. In particular, the EELS near edge structure can potentially provide information on the energy distribution of the empty electronic states above the Fermi level, the local density of states, oxidation state, and the bonding environment. For the actinides N_4,5 (4dâ?'5f) edge, the 4d5/2 and 4d3/2 peaks, or â?~white linesâ?T, are well separated by a large core spin-orbit splitting that is on the order of 40 eV. No fine structure is revealed because of the core-hole lifetime broadening. Moore et al. (2006, 2007) have examined this large splitting and probed the 5f spinâ?"orbit interaction directly for actinide metals. Selection rules govern that a d3/2 electron can only be excited into an unoccupied f5/2 level. This phenomenon has been used to measure the relative occupation of the 5f5/2 and 5f7/2 levels. The branching ratio (I(N₅)/[I(N₅)+I(N₄)] for a series of plutonium solids has been measured, including plutonium oxide precipitated on the surface of hematite and magnetite, as well as plutonium fluorides and phosphates. TEM and Electron diffraction data was collected to identify the various phases. The branching ratio analysis supports the appreciable covalent nature of the bonding in PuO₂.

The electronic structure of Pu materials is directly tied to the details of the 5f electron bonding and hybridization. In compounds one finds hybridization rather than direct 5f-5f bonding as the major contributor for 5f electron influence on electronic properties. We will show photoemission results for PuCoGa₅, PuCoIn₅, PuO₂, and Pu metal that span the range of interesting materials from Mott insulator to heavy fermion superconductor. The synergy between synthesis, spectroscopy and modeling provides an outstanding opportunity to explore details of the energy and crystal momentum dependence of Pu electronic structure. Through both angle-resolved photoemission on single crystal samples as well as temperature dependent photoemission, the electronic structure is measured and compared against advanced modeling based on theories beyond traditional density functional theory. The strength of the 5f electron hybridization may be quantified through dispersion in 5f electron peaks from the angle-resolved photoemission. In the case of PuO₂, we see over two eV of dispersion in the hybridized (O 2p - Pu 5f) valence band. For PuCoGa₅, the quasiparticle peak at the Fermi energy shows 50 meV or more of dispersion in reciprocal space over a range covering slightly less than half the zone center to zone boundary. These momentum-energy dispersions place significant constraints on models, which could be used to describe the electronic structure of these Pu materials. For PuCoGa₅, models which place the 5f electrons in a localized configuration without significant hybridization, would be inconsistent with experimental results. In the case of PuO₂, the dispersion measured in photoemission agrees well with the hybrid functional calculations and further support the increase in hybridization moving from ionic UO₂ to covalent PuO₂. In the case of Pu metal where polycrystalline samples are used, variations in photon energy give solid indication of substantial hybridization. New, high resolution (10 meV) data quantify the nature of the quasiparticle peak near the Fermi energy. The temperature dependence (10 to 300 K) of this peak also constrains viable models for the electronic structure of Pu metal. Work supported by the U.S. DOE Basic Energy Science, Materials Sciences and Engineering; LANL LDRD program, and Campaign 2.

Not only does plutonium offer a challenge for metallurgists through its myriad of structural chemistry ranging from low symmetry monoclinic room temperature structures to high symmetry and high temperature structures, but it also tests the physics community. One the biggest questions that needs to be answered is how to understand the true nature of the 5f electrons in Pu which have been shown to exhibit both itinerant (delocalized) or localized behavior depending on its electronic configuration. To circumvent the synthetic difficulties associated with the preparation of single crystals of several Pu allotropes, we can instead produce crystals of Pu intermetallics that can be directly compared to simple Pu binary compounds, e.g. PuIn₃ or PuGa₃, through a crystallographic analysis (similar coordination environments, bond distances, and building block units) as well as physical properties. We have recently grown single crystal samples of two new Pu ternary compounds that seem to be promising candidates for studying the duality of the 5f electrons of Pu. The compounds were isolated from metal flux growth reactions using their respective elements. The structure of PuPt₂In₇ is assembled from two simple building block moieties, PuIn₃ and PtIn₂ stacked alternatively along the c-axis. The resemblance of the structure to other related â?~115â?T or â?~218â?T compounds is apparent, which are all built from identical units of â?~1-3â?T and â?~1-2â?T moieties, derived from the AuBe₃ (PuIn₃ in previous) and CuMg₂ (PtIn₂ in previous) structure types, but differing in the stacking sequence. If the â?~115â?T compound can be represented as ABABAB stacking, where AuBe₃ is an A layer and CuMg₂ is a B layer then PuPt₂In₇ can be represented as ABBABBA. Single crystal X-ray diffraction reveals that the compound crystallized with the CePt₂In₇ structure type (space group I4/mmm) with unit cell parameters a = b = 4.5575(7) Ã. and c = 21.362(6) Ã.. No long range magnetic ordering is found for the compound however the heat capacity indicates heavy fermion-like behavior with a Sommerfeld coefficient of nearly 250 mJ/mol-K2. The structure of Pu₂PtGa₈ can be seen as yet another variant of the â?~115â?T type structure again just differing in the stacking arrangement of the PuGa₃ and PtGa₂ blocks. Single crystal X-ray diffraction reveals that the compound crystallized with the Ho₂CoGa₈ structure type (space group P4/mmm) with unit cell parameters a = b = 4.3408(4) Ã. and c = 11.076(2) Ã.. No long range magnetic order was found in this compound but the heat capacity measurements suggest heavy fermion behavior with a slightly enhanced Sommerfeld coefficient measuring 80 mJ/mol-K2. This presentation will touch upon the aspects of the structure/property relationships which can be drawn from new Pu-containing intermetallic compounds and how we can start to address some of the important issues surrounding the dual nature of Pu 5f electrons.