New high-pressure x-ray diffraction data on uranium metal (99.9 %) in a diamond anvil cell is presented to 100 GPa (Volume compression V/Vo = 0.700) at room temperature using a variety of pressure markers like ruby, copper, and platinum. The diffraction patterns are carefully indexed allowing for reversal of peak positions based on anisotropic compression. We report anisotropic compression of the orthorhombic unit cell with the axial ratio b/a increasing initially to 40 GPa followed by a rapid decrease at higher pressure. On the other hand, axial ratio c/a shows a rapid increase with increasing pressure followed by saturation at megabar pressures. The most recent full potential electronic structure calculations reproduce the increasing tend of axial ratio c/a to 100 GPa but do not explain the variation in the b/a ratio. Our detailed analysis of all available experimental data also indicates that the observed anisotropic effects are intrinsic to Uranium and are independent of the pressure medium used in the high-pressure experiments.

Research concerning the crystal chemistry of hexavalent U by the Environmental Mineralogy and Crystal Structures research group at Notre Dame has resulted in the description of more than 110 new structures of uranyl compounds (including 36 minerals). New insights into the crystal chemistry of U6+ are presented, with emphasis on recently discovered novel structural connectivities. The structural hierarchy of uranyl minerals and compounds, which was first established for 180 structures in 1996, has been extended to include 145 new structures. The hierarchy is based upon polymerization of polyhedra containing higher-valence cations, and consists of five distinct classes: structures containing isolated polyhedra (7), finite clusters of polyhedra (41), chains of polyhedra (52), sheets of polyhedra (184), and frameworks of polyhedra (41). The dominance of sheets in uranyl compounds (57% of known structures) arises from the unequal distribution of bond-valences within the uranyl polyhedra. Topological relations of the sheets in uranyl compounds are best understood by analysis of the topological distribution of anions within sheets in which sharing of polyhedral edges dominates, and by graphical representation of the connectivity of polyhedra in cases where sharing of vertices of polyhedra dominates the sheet.

Correlated-electron systems are so named due to strong interactions between electrons unlike traditional metals (e.g. copper) that have “free electrons” that interact very weakly. Knowledge of the Fermi surface, density of electron states and band structure are the starting points for a first-principles understanding of the electronic and electronically related macroscopic properties, e.g. equation of state. The use of high pressure and high magnetic fields to alter the electron-electron (hybridization) and electron-lattice interactions give us powerful tools to understand complicated rare earth and actinide correlated-electron systems and allows precise testing of experiment to theory. Correlated-electron systems yield a wide variety of ground states that are a direct result of the hybridization strength including: short and long range magnetic order, spin fluctuating, enhanced Pauli paramagnetism, heavy fermion behavior and superconductivity. We will review some results on U compounds in high magnetic fields and high pressures. By comparing the results to Ce compounds that have significantly more localized f electrons, the effect of direct 5f electron wavefunction overlap in U compounds can be discerned. Consequences on the search for U based heavy fermion superconductors will be discussed.

One effect of pressure on elements and compounds is to decease their interatomic distances, which can bring about dramatic perturbations in their electronic nature and bonding, which can be reflected in changes in physical and/or chemical properties. One important issue in the actinide series of elements is the effect of pressure on the 5f-electrons. We have probed changes in electronic behavior with pressure by monitoring structure by X-ray diffraction, and have studied several actinide metals and compounds from thorium through einsteinium. These studies have employed angle dispersive diffraction using synchrotron radiation, and energy dispersive techniques via conventional X-ray sources. The 5f-electrons of actinide metals and their alloys are often affected significantly by pressure, while with compounds, the structural changes are often not linked to the involvement of 5 f-electron. We shall present some of our more recent findings from studies of selected actinide metals, alloys and compounds under pressure. A discussion of the results in terms of the changes in electronic configurations and bonding with regard to the element's position in the series is also addressed.

Aging behavior and phase stability of a water-quenched U-6wt%Nb (U-14at%Nb) alloy artificially aged at 200°C and naturally aged at ambient temperature for 15 years have been investigated using Vickers hardness test, X-ray diffraction analysis, and transmission electron microscopy techniques. Age hardening/softening phenomenon is observed from the artificially aged samples according to microhardness measurement. The age hardening can be rationalized by the occurrence of spinodal decomposition, or fine scale of Nb segregation, which results in the formation of a nano-scale modulated structure within the artificially aged samples. Coarsening of the modulated structure after prolonged aging leads to the age softening. The occurrence of chemical ordering (disorder-order transformation) is found in the naturally aged sample based upon the observations of antiphase domain boundaries (APB's) and superlattice diffraction patterns. A possible superlattice structure is accordingly proposed for the chemically ordered phase observed in the naturally aged alloy sample.

Oxide thickness and composition averaged over a few square millimeter have been measured with nm thickness resolution by diffuse reflectance Fourier transform infrared (FTIR) spectroscopy. μ-Raman spectroscopy has been done on powders and bulk samples in the past, and can now be done on surfaces layers with μm lateral and depth resolution using con-focal microscopy. Here we apply con-focal-microscopy-based μ -Raman spectroscopy to a freshly polished/lightly oxidized and to heavily oxidized uranium to determine its sensitivity. The spectra show that μ-Raman spectroscopy does detect oxide thickness and oxide composition with high sensitivity.

The electronic structure of δ-phase Pu metal and the Pu-based superconductor PuCoGa5 is explored using photoelectron spectroscopy and a novel theoretical scheme. Excellent agreement between calculation and experiment defines a path forward for understanding electronic structure aspects of Pu-based materials. The photoemission results show two separate regions of 5f electron spectral intensity, one at the Fermi energy and another centered 1.2 eV below the Fermi level. A comparison is made between the photoemission data and five computational schemes for δ-Pu. The results for δ-Pu and PuCoGa5 indicate 5f electron behavior on the threshold between localized and itinerant and a broader framework for understanding the fundamental electronic properties of the Pu 5f levels in general within two configurations, one localized and one itinerant.

First-principles methods are employed to study the ground-state properties of δ-Pu-based alloys. The calculations show that an alloy component larger than δ-Pu has a stabilizing effect. Detailed calculations have been performed for the δ-Pu1−cAmc system. Calculated density of Pu-Am alloys agrees well with the experimental data. The paramagnetic → antiferromagnetic transition temperature (Tc) of δ-Pu100−cAmc alloys is calculated by the Monte-Carlo technique. By introducing Am into the system, one could lower Tc from 548 K (pure Pu) to 372 K (Pu70Am30). We also found that, contrary to pure Pu where this transition destabilizes δ-phase, Pu3Am compound remains stable in the antiferromagnetic phase that correlates with the recent discovery of the Curie-Weiss behavior of δ-Pu100−cAmc alloys at c ≥ 24 at. %.

Insight into the factors that control the formation and size of heavy metal phosphate nanophases in vitreous silica has been gained by combining conventional and anomalous small angle x-ray scattering studies with powder x-ray diffraction and laser-induced fluorescence investigations. Europium, thorium, and uranyl ions were sorbed from aqueous solutions into a chemically functionalized porous silica (termed Diphosil). Aliquots of those samples were heated to a series of temperatures that spanned the pore collapse point. Loading with trivalent europium ions resulted in production of nanophases whose size corresponded to the average number of metal ions per pore. Thorium or uranyl ions resulted in retention of porosity to higher temperature with eventual pore collapse that evidently resulted in formation of nanophases whose size exceeded that determinable under the experimental conditions.

Plutonium, because of its radioactive nature, ages from the “inside out” by means of self-irradiation damage and thus produces Frankel-type defects (vacancies and self-interstitial atoms) and defect clusters. The self-irradiation damage in Plutonium-239 occurs mainly by α-particle decay, where most of the damage comes from the U-235 recoil nucleus. The defects resulting from the residual lattice damage and helium in-growth could result in microstructural and physical property changes. Because these self-irradiation effects would normally require decades to measure, with a fraction (7.5 wt%) of Pu-238 is added to the reference plutonium alloy thus accelerating the aging process by approximately 18 times the normal rate. By monitoring the properties of the Pu-238 spiked alloy over a period of about 3.5 years, the properties of plutonium in storage can be projected for periods up to about 60 years. This paper presents density and volume changes observed from the immersion density and dilatometry measurements equivalent to aging the reference plutonium alloys to nine years.

By using electrochemical techniques in simultaneous combination with EXAFS, we can stabilize a targeted charge state of a solution complex and probe the coordination of selected actinide (An) ions. In addition, we can acquire sufficient XANES over a range of controlled electrochemical potentials, in the vicinity of a redox couple, to perform a Nernst analysis. The prospect of this technique to provide here-to-fore unattainable information is demonstrated through examples of Np, Pu, and Bk speciation in solution.

In delta phase Pu-Ga alloys, the transformation from the ductile face-centered cubic (fcc) δ phase that is retained at room temperature to the brittle low-temperature monoclinic alpha-prime phase is a thermally activated diffusionless transformation with double-c kinetics. Accurate modeling of the phase transformation requires detailed understanding of the role of plastic flow during the transformation and of the crystallographic transformation path. Using transmission electron microscopy (TEM), we find a significant increase in dislocation density in δ near the α′ plates, which suggests that plastic deformation contributes to the accommodation of the 20% reduction in volume during the transformation. Analysis of a series of optical micrographs of partially transformed alloys suggests that the α′ habit plane is usually nearly perpendicular to <111> δ. However, a small number of TEM observations support a habit plane near <112> or <123>, in agreement with earlier work.

We have constructed an experimental apparatus for the synthesis (via pulsed laser deposition) and analysis of nanoparticles and thin films of plutonium and other actinides. In-situ analysis techniques include x-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), scanning tunneling microscopy (STM), and scanning tunneling spectroscopy (STS). Also, the oxidation kinetics and the reaction kinetics of actinides with other gaseous species can be studied with this experimental apparatus. Preliminary results on depleted uranium are presented.

We have performed Photoelectron Spectroscopy and X-Ray Absorption Spectroscopy upon highly radioactive samples of Plutonium at the Advanced Light Source in Berkeley, CA, USA. First results from alpha and delta Plutonium are reported as well as plans for future studies of actinide studies.

Nuclear Magnetic Resonance studies of Ga stabilized δ-Pu reveal detailed information about the local distortions surrounding the Ga impurities as well as provides information about the local spin fluctuations experienced by the Ga nuclei. The Ga NMR spectrum is inhomogeneously broadened by a distribution of local electric field gradients (EFGs), which indicates that the Ga experiences local distortions from cubic symmetry. The Knight shift and spin lattice relaxation rate indicate that the Ga is dominantly coupled to the Fermi surface via core polarization, and is inconsistent with magnetic order or low frequency spin correlations.

Optical and photoelectron spectroscopies using VUV and Soft X-ray photons are powerful tools for studies of elemental and compound actinides. Large changes in the relative atomic cross sections of the 5f, 6d and sp electrons allow decomposition of the character of the valence bands using photoemission. Resonant enhancement of photoelectrons and Auger electrons at the 5d core threshold further aids the decomposition and gives a measure of elemental specificity. Angle-resolved photoemission can be used to map the momentum dependence of the electronic states. The large changes in relative cross section with photon energy yields further details when the mapping is done at equivalent points in multiple zones. Spectra for well understood rare earth materials will be presented to establish spectral characteristics for known atomic character initial states. These signatures will be applied to the case of USb to investigate f-d hybridization near the Fermi level.

The electronic structures of actinide solid systems are calculated using the self-interaction corrected local spin density approximation. Within this scheme the 5f electron manifold is considered to consist of both localized and delo-calized states, and by varying their relative proportions the energetically most favourable (ground state) configuration can be established. Specifically, we discuss elemental Pu in its δ-phase, and the effects of adding O to PuO2.

The effect of Am doping on the physical properties of three superconducting systems (RBa2Cu3O7, Pb2Sr2R1−xCaxCu3O8 and R2−xMxCuO4) have been studied using x-ray diffraction, x-ray absorption spectroscopy (XAS) and magnetic susceptibility experiments. Am is incorporated into all three systems as the tetravalent ion and superconductivity is not observed in any of the resultant compounds. The absence of superconductivity in Pb2Sr2Am0.5Ca0.5Cu3O8 is attributed to the transfer of charge from Am to the Cu-O planes. Systems that cannot incorporate a tetravalent ion, notably AmBa2Cu3O7 and Am2CuO4, do not form. Although Pr1.85Am0.15CuO4 forms with similar structure to Pr1.85Ce0.15CuO4, and Am like Ce is tetravalent, it is not superconducting. We argue that the absence of superconductivity in the Am doped Pr compound it is due to the hybridization of radially-extended f orbitals on the magnetic Am4+ ion with the Cu-O band states.

Here, we demonstrate the power of electron energy-loss spectroscopy (EELS) in a transmission electron microscope (TEM) to investigate the electronic structure plutonium. Using EELS, TEM, and synchrotron-radiation-based X-ray absorption spectroscopy (XAS), we provide the first experimental evidence that Russell-Saunders (LS) coupling fails for the 5f states of Pu. These results support the assumption that only the use of jj or intermediate coupling is appropriate for the 5f states of Pu. EELS experiments were performed in a TEM and are coupled with image and diffraction data, therefore, the measurements are completely phase specific. It is shown that EELS in a TEM may be used to circumvent the difficulty of producing single-phase or single-crystal samples due to its high spatial resolution.

We present results on the structural stability of PuGa alloys based on the local spin-density approximation and including non-local corrections to the exchange-correlation functional. We use the projector augmented wave (PAW) method to calculate the formation energies of five intermediate phases. The stability of the δ solid solution is analysed using selected supercells for total energy caclulations. Cluster variation calculations in the tetrahedron approximation are performed to study the effect of composition and of finite temperature on the ordering phenomena in the δ solid solution. The Pu-rich part of the Pu-Ga phase diagram is computed.