The electronic properties of bulk fcc americium and the (111) surface have been investigated with the full-potential linearized augmented plane wave (FP-LAPW) method as implemented in the WIEN2K suite of programs The study is carried out for the anti-ferromagnetic ground state of Am at different levels of theory: (1) scalar-relativity vs. full-relativity; (2) local-density approximation (LDA) vs. generalized-gradient approximation (GGA). Our results indicate that spin orbit coupling plays an important role in determining the electronic properties of both bulk fcc americium and the (111) surface. In general, LDA is found to give a higher total energy compared to GGA results. The spin orbit coupling shows a similar effect on the surface calculations regardless of the model, GGA versus LDA. The 5f localized-delocalized transition of americium is employed to explain our results. In addition, the quantum size effects in the surface energies and the work functions of fcc (111) americium ultra thin films (UTF) are also examined.

The high pressure behaviour of Cm metal is investigated theoretically using ab initio electronic structure methods. Our calculations reproduce the structural phase transitions which are obsereved in recent experiment performed by Heatman et al. [Science 309, 110 (2005)]. Calculated transition pressures are in reasonable agreement with the experiment. Calculations performed in antiferromagnetic state is essential to reproduce the stability of Cm-III phase. Thus stability of Cm-III phase is related to the spin polarization of 5f electrons.

An experimental apparatus designed for the synthesis, via pulsed laser deposition, and analysis of metallic nanoparticles and thin films of plutonium and other actinides was tested on depleted uranium samples. Five nanosecond pulses from a Nd:YAG laser produced films of ∼1600 Å thickness that were deposited showing an angular distribution typical of thermal ablation. The films remained contiguous for many months in vacuum but blistered due to induced tensile stresses several days after exposure to air. The films were allowed to oxidize from the residual water vapor within the chamber (2×10-10 Torr base pressure). The oxidation was monitored by in-situ analysis techniques including x-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and scanning tunneling microscopy (STM) and followed Langmuir kinetics.

As a consequence of the unusual nature of plutonium's electronic structure, point- and extended-defects are expected to, and do exhibit extraordinary properties. Low temperature magnetic susceptibility measurements on Pu and fcc-Pu(Ga) show that the magnetic susceptibility increases as a function of time, yet upon annealing the specimen returns to its initial magnetic susceptibility. This excess magnetic susceptibility (EMS) arises from the alpha-decay and U recoil damage cascades which produce vacancy and interstitials as point and extended defects. The temperature of the first annealing stage defines a temperature (<35K) below which we are able to characterize the time and temperature evolution of the accumulating damage cascades as being a saturation function. The temperature dependence of the EMS is well described by a time independent, Curie-Weiss curve arising from a volumetric region surrounding each U damage cascade. This saturation picture also leads directly to a determination of the microscopic volume of the specimen that is affected by the frozen-in damage cascade. For our measurements in δ-Pu we calculate a diameter of the magnetically affected volume of ∼250Å per damage cascade. This should be compared with an estimated volume that encloses the damage cascade itself (determined from molecular dynamics) of ∼100 Å. Hence, the ratio of these volumes is ∼8. The observed anomalous magnetic behavior is likely a consequence of the highly correlated nature of the electrons. Similarities with defects in hole-doped superconductors suggest a general phenomenon in strongly correlated electron systems, of which Pu may be a particularly unusual or special example.

Full-potential linearized-augmented-plane-wave (FP-LAPW) calculations for δ-Pu films up to seven layers at the ground antiferromagnetic state including spin-orbit coupling effects reveal that surface energy rapidly converges and the semi-infinite surface energy is predicted to be 1.18, 1.21, and 1.42 J/m2 for δ-Pu (111), (001), and (110) films, respectively. Density of states show that the 5f electrons of the three surfaces tend to be localized with a sequence of (111)→(001)→(110). It is also predicted that the work function of δ-Pu (110) films exhibits a quantum size effect up to seven layers, while the work functions of δ-Pu (001) and (111) films show some oscillations when the number of layers is less than five, while it becomes relatively stable when the number of layers is greater than five. In addition, the work functions are predicted to be 3.41, 3.11, and 2.99 eV for δ-Pu (111), (001), and (110) films at the ground state, respectively. Comparisons with available experimental and theoretical results in the literature show good agreement.

We have studied the crystal structure of the AmCm binary alloy under high pressure by means of first-principles self-consistent total-energy calculations using the generalized gradient approximation (GGA) for the density functional theory (DFT). The virtual crystal approximation (VCA) is used for the description of the alloy system. In the present study, we investigated the double hexagonal (P63/mmc) structure, the face centered cubic (Fm3m) structure, the face-centered orthorhombic (Fddd) structure and the primitive orthorhombic (Pnma) structure for the AmCm alloy. Antiferromagnetic calculations have been compared with ferromagnetic calculations for all these phases. Our results are in general good agreement with recent experiment performed by Lindbaum et al. [J. Phys.: Condens. Matter. 15, S2297 (2003)].

Wide-band reflectrometry and ellipsometry have been used to determine the optical properties n and k of freshly polished uranium and of the epitaxial oxide layer, and also the rate of oxide growth in air. Results for uranium metal as well as for epitaxial oxide are compared with single wavelength ellipsometry literature values.

We report pressure studies on the magnetic and superconducting properties of PuTGa5 (T=Co, Rh) and Americium metal down to 0.4K. Recent discovery of superconductivity in the Plutonium based compounds PuTGa5 (T=Co, Rh) has shed a new light on the study of Actinides compounds. The fact that these systems present lots of similarities with the CeTIn5 (T=Co, Rh, Ir) must not hide some fascinating features of these materials: they display superconducting characteristics one order of magnitude higher than their Cerium counterparts or all 5f Superconducting Heavy. Moreover they present at ambient pressure a Non Fermi Liquid described by a T4/3 law which seems to maintain up to the highest pressure achieved (∼20 GPa) for the 2 Plutonium superconductors. Finally we report superconducting pressure diagram of americium metal under pressure up to 27 GPa and down to 0.4K. The complex superconducting diagram Tc(p) related to the several phases transition led us to examine the localization delocalization aspect of the 5f electrons in this metal through the Mott transition.

Phase composition of the murataite-based ceramics containing 10 wt.% of mixed oxides simulating rare earth/actinide (REE/An) and actinide (An) fractions of high level waste (HLW) was studied. The ceramics were prepared by melting of oxide mixtures in Pt ampoules in air at ∼1500 °C. Ceramics with REE/An and An fractions surrogates are composed of predominant murataite-type phases and minor extra phases: perovskite and crichtonite. Three murataite-related phases with five- (5C), eight- (8C), and three-fold (3C) elementary fluorite unit cell are present in these ceramics. These phases form core, intermediate zone, and rim of the murataite grains, respectively. They are predominant host phases for the rare earth elements and uranium whose concentrations are reduced in a row: M-5C>M-8C>M-3C. Appreciate fraction of Ce, Nd, and Pu may enter the perovskite phase. In the An-Gd ceramic perovskite and murataite were found to be predominant and secondary in abundance phases respectively.

The structural phase stability of heavy f-electron metals is studied by means of density-functional theory (DFT). These include temperature-induced transitions in plutonium metal as well as pressure-induced transitions in the trans-plutonium metals Am, Cm, Bk, and Cf. The early actinides (Th-Np) display phases that could be rather well understood from the competition of a crystal-symmetry breaking mechanism (Peierls distortion) of the 5f states and electrostatic forces, while for the trans-plutonium metals (Am-Cf) the ground-state structures are governed by 6d bonding. We show in this paper that new physics is needed to understand the phases of the actinides in the volume range of about 15-30 Å3. At these volumes one would expect, from theoretical arguments made in the past, to encounter highly complex crystal phases due to a Peierls distortion. Here we argue that the symmetry reduction associated with spin polarization can make higher symmetry phases competitive. Taking this into account, DFT is shown to describe the well-known phase diagram of plutonium and also the recently discovered complex and intriguing high-pressure phase diagrams of Am and Cm. The theory is further applied to investigate the behaviors of Bk and Cf under compression.

Complexation of Np(V), U(VI) and Nd(III) with dimethyl-3-oxa-glutaramic acid (DMOGA) and tetramethyl-3-oxa-glutaramide (TMOGA) was studied in comparison with the complexation with oxydiacetic acid (ODA). Stability constants and enthalpy of complexation were determined by potentiometry, spectrophotometry and calorimetry. Thermodynamic parameters, in conjunction with structural information of solid compounds, indicate that DMOGA and TMOGA form tridentate complexes with the ether-oxygen participating in bonding with actinide/lanthanide ions. The trends in the stability constants, enthalpy and entropy of complexation are discussed in terms of the difference in the hydration of the amide groups and carboxylate groups and the difference in the charge density of the metal ions.

High energy x-ray scattering (HEXS) is becoming a valuable tool for quantifying actinide coordination environments in solution. The technique is described with and exemplified by a study on Th speciation. A comparison of data from two samples with pHs of 0 and 3.4 show differences in the Th structural environment that can be attributed to Th colloid formation at the higher pH. A further comparison of the HEXS data from the colloidal sample with Th L3-edge EXAFS data on the same sample clearly demonstrates the advantage of the scattering data when looking for correlations at second-near neighbor or longer distances.

Understandings of U transport at the Earth’s surface are quite important especially for the disposal of radioactive waste and the remediation of contaminated ground water. We examined long-term U transport in and around the Koongarra U deposit, Australia, which has been subjected to weathering for the past 2 million years. The formation of saléeite [Mg(UO2)2(PO4)2·10H2O] occurred at the rim of apatite [Ca5(PO4)3(F,OH)], where the ground waters had about 200 - 400 ppb U (∼10-7 mol·l-1) and were undersaturated with respect to saléeite. The laboratory experiments using apatite and U-containing solutions and Rutherford backscattering spectrometry of the apatite surface have suggested that a leached layer of about 100 nm thick is formed on dissolving apatite, saléeite is precipitated only in the leached layer by local saturation, and saléeite precipitated is added to the interface between the leached layer and solution. At lower U concentrations (∼10-8 mol·l-1), U occurred as nanocrystals (20-100 nm in size) of uranyl phosphates such as metatorbernite [Cu(UO2)2(PO4)2·8H2O], scattered between and attached firmly to nanocrystals (2-50 nm in size) of goethite and hematite. A possible mechanism is that U, P, and Mg or Cu adsorbed onto ferrihydrite form nanocrystals of uranyl phosphates during crystallization of goethite and hematite from ferrihydrite. The above two mechanisms significantly lower the U concentration in the ground water that flows into the Koongarra creek, 200 m downstream from the ore deposit, where the U concentration is ∼10-10 mol·l-1. We conclude that the two mechanisms of uranyl-phosphate mineralization control the long-term U transport at Koongarra.

Examples of uranium-oxide-based nanostructures are considered, including 2D organic-inorganic nanocomposites and nanotubules. In nanocomposites, interfacial interactions between organic and inorganic substructures can be studied by charge-density matching principle. Application of this principle to uranyl compounds requires special attention since surface area of uranyl-based 2D units is higher than that of other inorganic oxysalts units (i.e. metal phosphates). The charge-density matching principle is, however, observed either through tail interdigitation (for long-chain monoamines) or incorporation of acid-water interlayers into organic substructure (for long-chain diamines). In some compounds, protonated amine molecules form cylindrical micelles that involves self-assembly governed by competing hydrophobic/hydrophillic interactions. The flexible inorganic complexes present in the reaction mixture could then form around cylindrical micelles to produce highly undulated 2D sheets or nanotubules.

Plutonium, because of its radioactive nature, ages from the “inside out” by means of self-irradiation damage and thus produces nanoscale internal defects. The self-irradiation induced defects come in the form of Frenkel-type defects (vacancies and self-interstitial atoms), helium in-growth, and defect clusters. At present there are neither experimental nor theoretical models describing the changes in the electronic structure caused by the aging in Pu. This fact appears to be associated primarily with the absence of reasonably convincing spectroscopic evidence of the changes. This paper demonstrates that Resonant Photoemission, a variant of Photoelectron Spectroscopy, has strong sensitivity to aging of Pu samples. The spectroscopic results are correlated with an extra-atomic screening model, and are shown to be the fingerprint of mesoscopic or nanoscale internal damage in the Pu physical structure. This means that a spectroscopic signature of internal damage due to aging in Pu has been established.

Using first-principles density-functional theory calculations, we show that the anomalously large anisotropy of δ-plutonium is a consequence of greatly varying bond-strengths between the 12 nearest neighbors. Employing the calculated bond strengths, we expand the tenants of classical crystallography by incorporating anisotropy of chemical bonds, which yields a structure with the monoclinic space group Cm for δ-plutonium rather than face-centered cubic Fm3m. The reduced space group for δ-plutonium enlightens why the ground state of the metal is monoclinic, why distortions of the metal are viable, and has considerable implications for the behavior of the material as it ages. These results illustrate how an expansion of classical crystallography that accounts for anisotropic electronic structure can explain complicated materials in a novel way.

The plutonium α decay leads to the formation of numerous point defects in the metal structure. The multi-scale modeling of self-irradiation effects in plutonium alloys needs a quantitative knowledge of defects population properties. In this work, we initiated a parametric study of molecular dynamics displacement cascade simulations to get properties of defects microstructure such as number of point defects, number and size of clusters, spatial repartition and spatial expansion of the cascade. These data constitute some of the input parameters for the mesoscopic scale simulations. First results obtained for two 2 keV energy cascades simulations are presented and discussed.

The electronic structure of Pu materials is examined using photoelectron spectroscopy. For delta-phase Pu metal as well as PuCoGa5 and PuIn3, the 5f electrons appear to be at the threshold between localized and itinerant character. A mixed level model computational scheme is used which results in non-magnetic solutions for the electronic structure and agrees well with the photoemission measurements. Several other computational schemes are assessed against photoemission results for delta Pu. Additional insight is provided by O2 and H2 dosing of the delta Pu samples and consideration of surface effects. The experimental and computational results are consistent with the 5f electrons in Pu materials exhibiting a dual nature with some fraction of the 5f levels localized and not participating in the bonding while the other fraction of 5f character is involved in bonding and hybridization with the conduction electrons.

Phase composition of the murataite-based ceramics containing 10 wt.% ThO2, UO2, NpO2 or PuO2 and leaching of actinides using a MCC-1 procedure were studied. The ceramics were prepared by melting of oxide mixtures in Pt ampoules in air at ∼1500 °C. They are composed of predominant murataite-type phases and contain traces of extra phases (rutile, crichtonite, perovskite). At least two murataite-related phases with five- and eight-fold elementary fluorite unit cell (5C and 8C) were simultaneously observed. Minor phase 3C (murataite) in the ceramics doped with ThO2 and UO2 was also found. In the Th-bearing sample the 5C phase prevails over the 8C phase. In the U-bearing ceramic they co-exist in comparable amounts. The sample produced at 1500 °C contains crichtonite whereas the ceramic produced at lower temperature (1400 °C) contained rutile. Higher temperature favors further rutile reactions with formation of crichtonite. The Np- and Pu-doped ceramics are also composed of major the 5C and the 8C phases and minor rutile and crichtonite (in Pu-loaded sample only). Unlike the sample prepared under slightly re-ducing conditions (in glassy carbon crucible) the Pu-doped ceramic produced under neutral conditions (in Pt ampoule) doesn’t contain perovskite-type phase. Occurrence of perovskite in the first sample was sup-posed to be due to reduction of some Pu(IV) to Pu(III) during experiment. Leach rates (7-day MCC-1 test, 90 °C) of the actinide elements from all the ceramics studied are at the level of 10-6-10-7 g/(m2·day).

It was experimentally observed that a single Pu-2.0 at% Ga sample can be thermally cycled many times, with nearly the same amount of transformation and reversion during each cycle, provided that the sample is annealed at 375°C for 8 hours and then conditioned at 25°C for at least 6 hours. The ambient temperature conditioning period is crucial for obtaining the same amount of transformation to alpha-prime in each thermal cycle. Here, we present results from a series of experiments that investigate the effects of conditioning time and temperature on alpha-prime_formation. When the sample is conditioned at 25°C for times between 0 and 6 hours, the amount of alpha-prime formed is a function of conditioning time. For conditioning treatments between 6 and 70 hours, however, the amount of alpha-prime_formed is nearly constant. Conditioning treatments at –50°C, 150°C, and 370°C all resulted in less alpha-prime_formation than the 25°C treatments. We hypothesize that embryos of the thermodynamically stable alpha phase form during the conditioning treatments, and these embryos form alpha-prime upon subsequent cooling. This result is an indirect confirmation of the Russian Pu-Ga equilibrium phase diagram.