Actinide Materials Research Articles

Plutonium and quantum criticality

The unusual properties of the elemental plutonium have long been a puzzle. It has been suggested that these properties may be related to quantum criticality [G. Chapline, J.L. Smith, LA Sci. 26 (2000) 1]. In this talk we will describe some experimental observations on rare earth and actinide materials which suggest that there are pairing correlations in all f-electron metals, and that the anomalous properties of the actinides in the vicinity of Np/Pu/Am, even at elevated temperatures, is associated with a critical point in the variation of the pair condensate order parameter with atomic number.

Introduction to first-principles electronic structure methods: Application to actinide materials

This paper provides an introduction for non-experts to first-principles electronic structure methods that are widely used in condensed-matter physics. Particular emphasis is placed on giving the appropriate background information needed to better appreciate the use of these methods to study actinide and other materials. Specifically, the underlying theory is described in sufficient detail to enable an understanding of the relative strengths and weaknesses of the methods. In addition, the meaning of commonly used terminology is explained, including density functional theory (DFT), local density approximation (LDA), and generalized gradient approximation (GGA), as well as linear muffin-tin orbital (LMTO), linear augmented plane wave (LAPW), and pseudopotential methods. Methodologies that extend the basic theory to address specific limitations are also briefly discussed. Finally, a few illustrative applications are presented, including quantum molecular dynamics (QMD) simulations and studies of surfaces, impurities, and defects. The paper concludes by addressing the current controversy regarding magnetic calculations for actinide materials.

Localized and Itinerant States in Actinide Materials

AbstractThe electronic structure of single crystal UO2 and polycrystalline δ-Pu is examined using photoelectron spectroscopy. These two actinide materials exhibit properties consistent with the 5f electrons at the threshold between localized and itinerant character. The results for δ-Pu may be viewed as the 5f electrons 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. For UO2 where angle-resolved photoemission is available, one observes dispersion in the 5f features indicative of the 5f electrons being influenced by the periodic potential of the lattice rather than purely influenced by the site to which the 5f electrons are generally localized.

Plutonium and uranium accommodation in pyrochlore oxides

Fluorite-related ceramics have attracted considerable attention as potential host materials for the immobilisation of radionuclides. Here we have used computer simulation to investigate the relative solution energies and mechanisms for Pu3+, Pu4+ and U4+ accommodation in an extensive range of A2B2O7 pyrochlore compounds. Solution is considered from simple oxides and via co-solution mechanisms. Results are discussed in terms of their implications for actinide retention and materials fabrication.

Introduction to First-Principles Electronic Structure Methods: Application to Actinide Materials

AbstractThe purpose of this paper is to provide an introduction for non-experts to first-principles electronic structure methods that are widely used in the field of condensed-matter physics, including applications to actinide materials. The methods I describe are based on density functional theory (DFT) within the local density approximation (LDA) and the generalized gradient approximation (GGA). In addition to explaining the meaning of this terminology I also describe the underlying theory itself in some detail in order to enable a better understanding of the relative strengths and weaknesses of the methods. I briefly mention some particular numerical implementations of DFT, including the linear muffin-tin orbital (LMTO), linear augmented plane wave (LAPW), and pseudopotential methods, as well as general methodologies that go beyond DFT and specifically address some of the weaknesses of the theory. The last third of the paper is devoted to a few selected applications that illustrate the ideas discussed in the first two-thirds. In particular, I conclude by addressing the current controversy regarding magnetic DFT calculations for actinide materials. Throughout this paper particular emphasis is placed on providing the appropriate background to enable the non-expert to gain a better appreciation of the application of first-principles electronic structure methods to the study of actinide and other materials.

Actinide Materials Research Supported by the Office of Basic Energy Sciences, U.S. Department of Energy

AbstractFor Abstract see ChemInform Abstract in Full Text.

Preparation of targets by electrodeposition for heavy element studies

For heavy element studies at GSI, lanthanide and actinide targets have been prepared by molecular plating. The deposition occurs from an isopropanolic solution at 1000–1200 V with current densities of a few mA/cm 2. Several lanthanide targets have been prepared for test experiments. With natGd deposited on a 10 μm thick Be backing foil a target density of 1100 μg/cm 2 could be achieved. Gd-targets were used for the production of α-emitting isotopes of Os, the homologue of hassium (Hs; Z=108), in order to develop a chemical separation procedure for Hs. 248Cm targets with densities up to 730 μg/cm 2 have been produced for recent experiments to investigate the chemical behaviour of Hs. Here, a rotating wheel system with a multi-target device has been applied enabling higher beam intensities, compared to a stationary target. The targets were irradiated with a pulsed 26Mg 5+ beam applying beam currents up to 6.6 μA electr. An α-spectroscopic investigation of the irradiated Cm-targets showed that the Cm-material is not evenly distributed over the entire target area. Very often, for heavy element investigations, chemical separation procedures are required to ensure high purity of the deposited actinide materials.

Actinide Materials Research Supported by the Office of Basic Energy Sciences, U.S. Department of Energy

ABSTRACTThe fundamental research topic in actinide chemistry and materials sciences is the role that the 5f electrons play in bond formation, which provides the central focus for DOE BES-supported actinide science. Structural systematics of the actinide metals, oxides, and other compounds as a function of atomic number are well established. Magnetic measurements have shown that the light actinide metals have delocalized 5f orbitals (i.e., the 5f electrons form bands), whereas the f electrons become localized at americium. Thus, the magnetic behavior of the first part of the actinide series resembles that of the d transition metals whereas the heavier actinides exhibit behavior similar to the rare earth metals. Spectroscopic results have established electronic energy levels, crystal field splitting, and near-neighbor coordination. The 5f orbitals participate in the band structure of materials that contain the light actinide metals and some of their intermetallic compounds, and perhaps in molecular compounds. Molecular-level information on the geometry and bonding in solids, at surfaces, and in clusters can now be obtained at BES-supported facilities.

Soft X-ray Synchrotron Radiation Investigations of Actinide Materials Systems Utilizing X-ray Emission Spectroscopy and Resonant Inelastic X-ray Scattering

ABSTRACTSynchrotron radiation (SR) methods have been utilized with increasing frequency over the past several years to study topics in actinide science, ranging from those of a fundamental nature to those that address a specifically-targeted technical need. In particular, the emergence of microspectroscopic and fluorescence-based techniques have permitted investigations of actinide materials at sources of soft x-ray SR. Spectroscopic techniques with fluorescence-based detection are useful for actinide investigations since they are sensitive to small amounts of material and the information sampling depth may be varied. These characteristics also serve to simplify both sample preparation and safety considerations. Examples of investigations using these fluorescence techniques will be described along with their results, as well as the prospects for future investigations utilizing these methodologies.

X-ray diffraction analysis of phase formation in synthesis of actinide matrices

The products of synthesis of crystalline matrices promising for immobilization of actinides were studied by the method of x-ray phase analysis. The experiments were performed on compositions corresponding to the phase stoichiometry of structural types of zirconolite (CaZrTi2O7 ) and pyrochlore (CaCeTi2O7 ,G d 2Ti2O7, Gd2Zr2O7 ). The experiments were carried out within a temperature range of 800 – 1600°C for a sintering time of 5 – 50 h, in air and in an oxygen temperature. The phase formation conditions in matrices of different compositions are identified. Practical recommendations are issued. Oxides with a lattice of the fluorite type (cubic zirconium dioxide, zirconolite, pyrochlore) have a high capacity with respect to actinides and high chemical and radiation resistance [1 – 4]. Therefore, it is advisable to use such phases as matrices, in which waste actinide materials can be shipped to burial or transmutation. This method of utilization of actinides is related to the development of methods of synthesis of such materials. One commonly used method is cold pressing — sintering (CP-S). It was used in the USA to develop a technology of immobilization of so-called “superfluous” plutonium into a titanate with the pyrochlore structure [4]. In optimizing the synthesis parameters it is essential to determine the temperature and duration of the process to ensure the production of a material with the required properties. For this purpose we investigated the kinetics of phase formation with lattices of the zirconolite and pyrochlore types in CP-S synthesis. The compositions of samples consisted of the following series of elements: Ca – Zr – Ti – O, Ca – Ce – Ti – O, Gd – Ti – O, and Gd – ZrO – O. The target phases were zirconolite (the ideal formula is CaZrTi 2 O 7 ) or pyrochlore (CaCeTi 2 O 7 ,G d 2 Ti 2 O 7 ,G d 2 Zr 2 O 7 ). The initial batches were prepared from carbonate (CaCO 3 ) and oxides (ZrO 2 ,T iO 2 , CeO 2 ,G d 2 O 3 ) crushed in an agate mortar to a size of 20 –3 0m. The mixtures were molded under a pressure of 200 – 400 MPa in the form of tablets of diameter 15 – 20 mm and height 2–4m m andsintered in alundum crucibles in a resistance furnace. The experiments were carried out in air and the experiments with the compositions containing cerium were performed as well in an oxygen atmosphere. Altogether more than 100 samples were prepared and analyzed using a Philips PW304000 X’Pert MPD diffractometer in the following conditions: CuK radiation, voltage 40 kV, cur

Strong coupling between local moments and superconducting 'heavy' electrons in UPd2Al3.

The electronic structure of heavy-fermion compounds arises from the interaction of nearly localized 4f- or 5f-shell electrons (with atomic magnetic moments) with the free-electron-like itinerant conduction-band electrons. In actinide or rare-earth heavy-fermion materials, this interaction yields itinerant electrons having an effective mass about 100 times (or more) the bare electron mass. Moreover, the itinerant electrons in UPd2Al3 are found to be superconducting well below the magnetic ordering temperature of this compound, whereas magnetism generally suppresses superconductivity in conventional metals. Here we report the detection of a dispersive excitation of the ordered f-electron moments, which shows a strong interaction with the heavy superconducting electrons. This 'magnetic exciton' is a localized excitation which moves through the lattice as a result of exchange forces between the magnetic moments. By combining this observation with previous tunnelling measurements on this material, we argue that these magnetic excitons may produce effective interactions between the itinerant electrons, and so be responsible for superconductivity in a manner analogous to the role played by phonons in conventional superconductors.

Magnetic ordering in strongly correlated-electron uranium systems: Consequences of two kinds of f-electron-band–electron states

Magnetic ordering involves the electronic behavior globally; and for uranium-based systems, the hybridization-induced effects dominate over the Coulomb exchange effects in determining the magnetic ordering. Therefore, as long as the hybridization is treated as acting between properly exchange-symmetrized two-electron wave functions, the effects of exchange can be incorporated in the one-electron exchange-correlation potential. As a consequence of the necessary exchange symmetrization, there are essentially two kinds of f electrons, localized magnetic and itinerant nonmagnetic. This has enabled us to make absolute material-specific predictions of alloying or high-pressure effects on magnetic ordering in uranium strongly correlated-electron (SCE) systems using local-density approximation input into many-electron dynamics. Experimentally, the alloying effects can be dramatic, e.g., in UxLa1−xS the magnetic ordering abruptly disappears at about 55% uranium. The theory is quite successful in its detailed absolute predictions, and this has important implications for the overall understanding of electronic behavior in SCE systems including heavy fermion systems. The key conclusion is that strengthening the hybridization, as kinematically restricted by exchange symmetry, leads to a chemical-environment-dependent sharp phase transition in SCE systems with dramatic observable consequences. This phase transition is associated with the elimination of the localized-magnetic transition-shell electrons (f electrons for light actinide and cerium-based SCE materials, d electrons for transition-metal–oxide-based SCE materials).

Construction of the JAERI soft X-ray beamline for actinide material sciences.

An undulator beamline for spectroscopy studies focusing on the electronic structure of actinide materials is under construction. Linearly or circularly polarized soft X-rays are provided by employing a variably polarizing undulator. Varied-line-spacing plane gratings and a sagittal-focusing system are used to monochromatize the undulator beam, whose energy ranges from 0.3 to 1.5 keV. A resolving power of 10(4) is expected in the whole energy region. These components are methodically operated by the SPring-8 beamline control system. There are three experimental stations in the beamline. In one of the stations the photoemission spectroscopy experiments are carried out at a radioisotope-controlled area where actinide compounds as well as unsealed radioactive materials are usable. Other experimental stations are planned in the beamline for surface photochemical reactions and biological applications.

ChemInform Abstract: Correlated Band Magnetism of Cerium and Actinide Materials

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Deviations from Fermi-liquid behaviour in transition-metal alloys

Data on the specific heat of Rh-Fe alloys and on the resistivity of these and other 4d-3d alloys are presented to show that deviations from Fermi-liquid behaviour of the type that have been reported and discussed for actinide and rare-earth materials are also found at compositions close to those for the onset of a magnetic ground state in alloys of 3d transition metals. It is suggested that a unique class of non-Fermi liquids is unlikely.

The high-temperature vaporization/decomposition of actinide materials: Fundamental and technological aspects

The unique and variable chemistry exhibited across the actinide series offers a scientifically challenging group of elements. Investigations of their high-temperature materials science and thermodynamics can provide important insights into fundamental actinide science and generate critical information for technological applications. The wide variations in the physicochemical properties and the availability of the various actinide elements necessitates special experimental techniques be employed, even when investigating a single property (e.g., the enthalpy of vaporization). Experimental approaches range can from classical to novel methods to permit studies of the rarer and the more intensely radioactive members. We have developed special equipment and techniques to investigate the high-temperature, physicochemical properties of several actinides through Md, as well as the precursor elements, radium and actinium. These experimental approaches employed both alloys and solid solutions of compounds, in addition to the pure elements themselves. The vaporization/decomposition of compounds and the sublimation of the elements, together with the associated thermodynamics, are the main issues. Results for several of the actinide elements, some of the techniques employed and systematic comparisons of the thermochemical aspects of bonding are discussed to give a fuller understanding of the series as a whole.

Correlated band magnetism of cerium and actinide materials

We discuss (1) the effects to be expected by the introduction into the electronic structure of locally-based two-electron correlations between the f electrons and bonding electrons of p and d atomic origin centered off-site as well as f-f correlations, (2) the expected observable consequences of these two-electron correlations, and (3) how to perform electronic structure calculations including the two-electron correlations. We first review certain general features of the physics associated with capturing the dual energetically localized-delocalized nature of the f electron spectral density; and review model calculations involving a single on-site f electron and a single ligand p/d electron of off-site parentage which lead to the possibility of a narrow singlet and triplet (magnetic) band picture explaining heavy fermion phenomenology. We then show that the same singlet/magnetic state picture arises when we include two-electron f-l and f-f correlations for actinides, which have atomic fn configurations with n>1; and we describe a practical electronic structure scheme for real materials based on a sequence in which a conventional one-electron linearized combination of muffin-tin orbitals (LMTO) LDA+U calculation is followed by a calculation for the lattice with a helium like two-electron Hamiltonian at the f atom sites, i.e., two-electron atoms where initially for the core two electrons worth of charge are removed from the LMTO f-site atom. This procedure will reconstruct the LMTO bands to include two-electron texturing.

Photoemission and inverse photoemission studies on actinide materials--does any model work?

An overview is given of issues in the comparison of theory and experiment for actinide 5f electron spectra, with new experimental results on the photon energy dependence of 5f photoemission in UAl 2 and the 4f core levels of the four phases of Pu metal.

Structural and physical properties of actinide materials at high pressure

Selected results of in situ high pressure studies of actinide metals and compounds are described. A comparison is attempted between the pressure-induced structural phase transitions of UAs, UO 2 and Th 3P 4 and the optical response of these compounds at high pressure. The structural properties of UTe, USe and PuSb at high pressure are compared with their electrical and magnetic properties. In certain cases a clear correlation could be established between a structural phase transition at room temperature and changes in reflectivity at the same temperature or in electrical resistance at temperatures between room temperature and 1.3 K. However, for some of the materials discussed the evolution of the physical properties is largely independent of the structural phase transitions.

Resonant magnetic scattering from 5f magnetic materials (invited)

Over the last five years, the large resonant enhancements of the magnetic x-ray scattering amplitudes at the MIV and MV absorption edges of actinide materials (U, Np) have been exploited in a series of magnetic diffraction investigations at the National Synchrotron Light Source. This work is reviewed with a discussion of measurements on a series of antiferromagnetic materials with contrasting magnetic properties, UAs, USb, U0.85Th0.15Sb, UO2, URu2Si2, and NpAs.