9:45 AM - **Z1.1
Soft X-ray Synchrotron Radiation Investigations: Current X-ray Emission Spectroscopy and Scanning Transmission X-ray Microscopy Studies of Actinides.
David Shuh 1 , Per Anders Glans 2 , Jinghua Guo 2 , Tolek Tyliszczak 2 , Sergei Butorin 3 , Kristina Kvashnina 3 , Joseph Nordgren 3 , Lars Werme 3 , Tsuyoshi Yaita 4
1 Actinide Chemistry Group, The Glenn T. Seaborg Center, Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 2 Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 3 Department of Physics and Materials Science, Uppsala University, Box 530, Uppsala Sweden, 4 Actinide Coordination Chemistry Group, Synchrotron Radiation Research Center, Quantum Beam Science Directorate, Japan Atomic Energy Agency (JAEA), 1-1-1Koto, Sayo-cho, Sayo-gun, Hyogo Japan
Show AbstractSoft x-ray synchrotron radiation (SR) methodologies are being developed and utilized at the Advanced Light Source (ALS) of Lawrence Berkeley National Laboratory (LBNL) to investigate a range of issues in actinide science. The techniques of near-edge x-ray absorption fine structure (NEXAFS) spectroscopy, x-ray emission spectroscopy (XES), and resonant inelastic x-ray scattering (RIXS) are the core approaches currently being utilized. An advantage of soft x-ray SR approaches is the capability to directly probe the K-edges of light elements, other low-lying thresholds, the actinide 5d core levels with high resolution, and the increasing capability of theory to complement the results from soft x-ray SR experiments. Competing with the utility of soft x-ray SR techniques are challenging technical safety considerations and prerequisite difficulties in sample preparation for soft x-ray experiments. The photon-in, photon-out nature of XES/RIXS makes for tractable sample preparation and are excellent probes of actinide f-electron character, oxidation state, and electronic structure when coupled to NEXAFS and theory. The most recent results from fundamental RIXS investigations of Pu, Am, and Am materials will presented. Complementing NEXAFS measurements, comparative XES studies of lanthanide and actinide complexation by a suite of soft donor ligands designed for trivalent separations has proven extremely useful to understand and rationalize the preferential selectivity of ligands based on their characteristics. For NEXAFS investigations, scanning transmission x-ray microscopy (STXM) spectromicroscopy at the ALS-Molecular Environmental Science (MES) Beamline 11.0.2 has been utilized to investigate a diverse range of actinide science. The ALS-MES STXM is capable of imaging particles with a spatial resolution better than 30 nm, can directly probe the light element K-edges below 2 keV, and utilizes the actinide 4d core level to obtain spectroscopic information from actinide constituents. The results from current investigations that demonstrate the unique capabilities of the STXM for actinide science will be highlighted. Based on and in light of current soft x-ray SR experiences, the prospects and potential for future actinide studies in the soft x-ray region, as well as the disadvantages, will be critically discussed. Iportantly, the mechanisms and infrastructure necessary to conduct enhanced soft x-ray investigations in the future will be addressed.
12:15 PM - Z2.4
Reactions of Dipositive Actinide Ions With Small Alkanes: A Combined Theoretical and Experimental Study.
Emanuela Di Santo 1 , Maria del Carmen Michelini 1 4 , Joaquim Marcalo 2 , Marta Santos 2 , Antonio Pires de Matos 2 , Richard Haire 3 , Nino Russo 1 , John Gibson 4
1 Dipartimento di Chimica, Università della Calabria, Arcavacata di Rende Italy, 4 Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 2 Unidade de Ciências Químicas e Radiofarmacêuticas, Instituto Tecnológico e Nuclear, Sacavém Portugal, 3 Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show AbstractThe activation of small alkanes and alkenes by bare monopositive actinide ions has been studied both experimentally [1-4] and theoretically [5,6]. The information gleaned from those studies has been very valuable in addressing central questions regarding the chemical behaviour of actinide ions in the gas phase. Preliminary qualitative experimental results for some reactions of dipositive actinide ions with alkanes and alkenes have been reported [7]. As part of an ongoing project on the H-C and C-C bond activation of alkanes and alkenes by gas phase bare actinide ions [8], we present here the reaction pathways for the interaction of early bare dipositive actinide ions with small alkanes. A detailed description of the reaction mechanisms leading to different dissociation channels was obtained using Density Functional Theory (DFT). The theoretical approach was chosen based on the performance observed in previous studies of reactions involving actinide cations and small molecules. The theoretical calculations are compared with experimental results where the reactions were investigated using Fourier transform ion cyclotron resonance mass spectrometry [8]. This experimental technique was recently employed to study reactions of dipositive lanthanide ions with alkanes and alkenes [9].In general, good agreement was observed between the experimental and computed results, which provides confidence that these DFT approaches can reliably model such reaction mechanisms.*This work was supported by the Università degli Studi della Calabria; by Fundação para a Ciência e a Tecnologia (FCT) under contract PPCDT/QUI/58222/2004; and by the Director, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences of the U.S. Department of Energy under Contract DE-AC02-05CH11231 at LBNL.[1] Heinemann C, Cornehl H H and Schwarz H 1995 J. Organomet. Chem. 501 201. [2] Marçalo J, Leal J P and Pires de Matos, A 1996 Int. J. Mass Spectrom. and Ion Processes 157/158 265.[3] Gibson J K, 2002 Int. J. of Mass Spectrometry 214 1. [4] Gibson J K, Haire R G, Marçalo J, Santos M, Pires de Matos A, Mrozik M K, Pitzer R M and Bursten B E 2007 Organometallics 26 3947. [5] Di Santo E, Michelini M C and Russo N 2009 Organometallics 28 3716.[6] Di Santo E, Michelini M C and Russo N 2009 J. Phys. Chem. A (web-published Sept. 01, 2009).[7] Gibson J K, Haire R G, Marçalo J, Santos M, Leal J P, Pires de Matos A, Tyagi R, Mrozik M K, Pitzer R M and Bursten B E 2007 Eur. Phys. J. D 45, 133. [8] Di Santo E, Michelini M C, Marçalo J, Santos M, Pires de Matos A, Haire R G, Russso N and Gibson J K 2009 (unpublished results).[9] Marçalo J, Santos M, Pires de Matos A, Gibson J K and Haire R G 2008 J. Phys. Chem. A 112, 12647.
Symposium Organizers
Scott McCall Lawrence Livermore National Laboratory
Eric Bauer Los Alamos National Laboratory
John Gibson Lawrence Berkeley National Laboratory
Thomas Fanghaenel European Commission Joint Research Center
Lynda Soderholm Argonne National Laboratory
Z5: Chemistry of Nuclear Fuels
Session Chairs
Wednesday AM, April 07, 2010
Room 3008 (Moscone West)
9:30 AM - Z5.1
Simulations of the Miscibility of Urania and Tetrahedral Zirconia Subjected to Track Damage.
Steve Valone 1 , Xian-Yang Liu 1
1 Materials Science and Technology Division, LANL, Los Alamos, New Mexico, United States
Show AbstractSome advanced nuclear fuel concepts are based on composites of fissile and nonfissile materials. One example might be uranium oxide fuel dispersed with an inert matrix material such zirconium dioxide. The concept behind such designs is that the nonfissile phase will capture a significant fraction of the fission products and minor actinides through geometric arrangement of the materials or through gettering of the fission product species in the nonfissile material. The success of such strategies will depend in some measure on the immiscibility of the fissile-nonfissile materials under irradiation. Miscibility depends on the phases of the materials. During operation in a nuclear reactor, fission tracks will cross these interfaces heating both materials. Heating may possibly induce phase transformation of one of the materials. Here we focus on urania in contact with pure zirconia. Urania is relatively immiscible in zirconia when the zirconia is in its monoclinic or tetrahedral phases [1, 2]. However, in the high-temperature fluorite phase, urania is completely miscible in zirconia. The behavior of these material interfaces is simulated through molecular dynamics. Simulation cells are composed of bilayers urania in the fluorite structure and zirconia in the tetrahedral structure. Simulations of interfaces heated from thermal spikes and heated interfaces, subjected to cascade damage, will be discussed. We use established potentials for both materials in their fluorite phases [3, 4]. Tetragonal distortions are introduced into the zirconia potential in a manner analogous to the modified embedded atom potential [5] from electronic structure information [6]. [1] E. C. Subbarao: Advances in Ceramics 3, Science and Technology of Zirconia I, (Amer. Ceram. Soc., Columbus, Ohio, 1981) p. 1. [2] M. Yashima, T. Koura, Y. Du, M. Yoshimura, J. Amer. Cer. Soc. 72, 512 (2005). [3] R. W. Grimes and C. R. A. Catlow, Phil. Trans.: Phys. Sci. Engin. 335, 609 (1991). [4] J. Yu, R. Devanathan and W. J. Weber, J. Mater. Chem. 19, 3923 (2009). [5] M. I. Baskes, Phys. Rev. B 46, 2727 (1992). [6] H. J. F. Jansen, Phys. Rev. B 43, 7267 - 7278 (1991).
9:45 AM - Z5.2
Theoretical Studies of Xe Redistribution in UO2±x.
David Andersson 1 , Blas Uberuaga 1 , Pankaj Nerikar 1 , Neil Carlson 1 , Chris Stanek 1
1 , Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Show AbstractSpecies diffusion is a critical process in nuclear fuels and it is strongly coupled to microstructure evolution via for example fission gas redistribution and progression of the oxygen to metal ratio across the fuel pin. Using density functional theory (DFT) calculations, molecular statics calculations and meso-scale continuum simulations we have studied Xe redistribution in UO2±x. In this talk we will summarize these efforts and provide examples from each type of simulation, ultimately focusing on predicting Xe fission gas release as function of the oxygen stoichiometry in UO2±x.The first controlling step for fission gas release is diffusion of individual gas atoms through the fuel matrix to existing sinks (bubbles, grain boundaries and dislocations), a process that is governed by the activation energy for bulk diffusion of gas atoms, the driving force for the sink segregation and the sink saturation limit. Xe bulk diffusion mechanisms are studied by calculating the corresponding activation energies as function of the UO2±x stoichiometry using density functional theory (DFT) methods. This involves determining Xe migration barriers as well as thermodynamics of Xe trap sites in UO2±x and their interactions with uranium vacancies that enable Xe in trap sites to move. We then use results from atomistic simulations of the grain boundary sink strength as function of the Xe concentration to formulate a thermodynamic model of Xe grain boundary segregation. The Xe mobilities calculated from DFT and the Xe segregation models are used as starting point for deriving a transport model that explicitly accounts for the Xe interaction field with grain boundaries. In particular this model distinguishes between different types of boundaries. The transport model is then solved for both idealized and realistic grain boundary microstructures within a finite element code.
10:00 AM - Z5.3
Mixing Energetics and Charge Transfer Reactions in the (U,Ce)O2 System.
Benjamin Hanken 1 , Mark Asta 1 , Niels Gronbech-Jensen 2 , Alexandra Navrotsky 1 3
1 CHMS, UC Davis, Davis, California, United States, 2 Applied Science, UC Davis, Davis, California, United States, 3 Peter A. Rock Thermochemistry Laboratory and NEAT ORU, UC Davis, Davis, California, United States
Show AbstractThe thermophysical properties of (U,Pu)O2, the basis for mixed oxide (MOX) nuclear fuels, are a topic of renewed interest due in part to the material’s potential applicability in the next-generation nuclear reactor, the sodium fast reactor. In particular, as the oxide is doped with actinides, the changes to its thermodynamic and physical properties at high temperatures must be well understood for these fuels to be safely used in an advanced fuel cycle. Due to the difficulties associated with the handling of plutonium, cerium is often used as a surrogate in experimental studies. Another possible avenue for studying MOX is through computational modeling, employing first-principles methods based on density functional theory (DFT). However, it is necessary to go beyond standard DFT methods to properly account for the localized nature of the f-electrons. With the ability to investigate mixing thermodynamics of (U,Ce)O2 using high temperature drop solution calorimetry at UC Davis, this system is a natural candidate by which to benchmark such computational approaches to the study of mixing thermodynamics of mixed-oxide fuels. The GGA+U functional has been applied to the fluorite structured (U,Ce)O2 system to determine mixing energetics of various ordering schemes. The results reveal a competition between two different charge states, one characterized by 4+ charges on the cations, the second resulting from charge transfer between Ce and U ions, leading to the presence of reduced Ce(III) and oxidized U(V) ions. While the mixing energies for the first are nearly ideal, the second displays non-ideal mixing enthalpies for reasonable choices of the Hubbard-U parameter in the GGA+U method. The overall trends obtained with the GGA+U approach are shown to be well described by classical, polarizable-shell ionic potential models. The pair-potential models are used in larger-scale simulations to investigate the energetics of both ordered and disordered configurations, with varying degrees of charge mixing. The results provide a framework for interpreting origins of non-ideal solution thermodynamic properties which will be probed by oxide melt solution calorimetry.
10:15 AM - Z5.4
First Principles Calculations of Diffusion Properties of Actinide Oxide Fuel Materials.
Jianwei Wang 1 , Udo Becker 1
1 Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan, United States
Show AbstractSpent nuclear fuel from commercial reactors is composed of 95-99% UO2 and transuranium oxides. These oxides are expected to form solid solutions with UO2, and their stability strongly affects reprocessing and waste materials. Therefore, it is important to determine if potential phase separations might take place or if ordered phases from these solid solutions might form. Due to difficulties in obtaining experimental values of thermodynamic data from calorimetric data or obtaining structural information of ordered phases, we use first-principles calculations to understand solid-solution properties of actinide oxides and of diffusion properties of actinides in UO2. The focus is the structure, energetics, diffusion of cations, and the kinetics of exsolution and ordering processes.For static calculations, the Transition State Theory is applied to calculate diffusion properties of actinide elements in UO2 crystal matrix. The activation barrier and attempt frequency are calculated based on quantum mechanical calculations, using the Density Functional Theory (DFT) framework and planewave basis sets as implemented in the VASP package. On-site Coulomb interaction, fully relativistic calculation for the core-electrons and scalar relativistic approximation for the valence electrons, and spin-orbit interactions using the second-variational method are employed to reasonably account relativistic effects and electron correlation for actinide elements. For dynamics simulations, empirical potential and ab initio molecular dynamics simulations at high temperatures are also performed to simulate the diffusion process directly.Structure, energetics, and diffusion properties of Th, U, Np in UO2 are studied as a function of spin configuration, and preexisting point defects. In addition, different diffusion pathways are investigated. The results show that electronic configurations of these three elements (Th, U, Np) and their spin states have significant influence on the energetic barrier and attempt frequency of the diffusion process. Preexisting defects and reaction paths also strongly influence the diffusion properties. Bonding environment, localization and delocalization of 5f electrons, and charge state of the three elements in UO2 are examined and their connection to the diffusion properties is discussed.
10:30 AM - Z5.5
Atomistic Simulations of Thermal Conductivity Variation in Oxide Nuclear Fuel.
Christopher Stanek 1 , Pankaj Nerikar 1 , Tim Germann 1 , Blas Uberuaga 1 , Ken McClellan 1 , Susan Sinnott 2 , Simon Phillpot 2
1 Material Science and Technology, Los Alamos National Laboratory, Los Alamos, New Mexico, United States, 2 Materials Science and Engineering Department, University of Florida, Gainesville, Florida, United States
Show AbstractUranium dioxide (UO2) is the standard nuclear fuel in pressurized water reactors. Fission gases such as xenon (Xe) migrate to grain boundaries and cause swelling of the fuel. Both fission products and grain boundaries contribute to the thermal resistance and affect the thermal conductivity of nuclear fuels. The influence of grain boundaries and fission product composition on thermal conductivity in UO2 is explored in this work using molecular dynamics (MD) simulations. The specific boundaries studied were symmetric Σ5 tilt, Σ5 twist, and an amorphous boundary. In order to accurately consider the variation of thermal conductivity as a function of fission product composition and microstructure, we simulated a computationally challenging number of atoms (1E6). We also modified an existing MD code (SPaSM) to simulate ionic materials. Possible implications for providing an improved input in to meso and continuum scale models through a fundamental understanding of the lower length scale phenomena that govern variations in thermal conductivity are discussed. We will also importantly discuss the results of complementary laser flash analysis experiments.
10:45 AM - Z5: ChemFuel
BREAK
Z6: Actinide Chemistry II
Session Chairs
Wednesday PM, April 07, 2010
Room 3008 (Moscone West)
11:15 AM - Z6.1
Actinide Correlations in Solution and their Relative Influence on Measured Energetics.
Lynda Soderholm 1 , S. Skanthakumar 1 , Richard Wilson 1
1 Chemical Sciences and Engineering, Argonne National Laboratory, Argonne, Illinois, United States
Show AbstractLigating anions and solute molecules form f-ion complexes that influence the solubility, stability and reactivity of the metal ion. Although there are a variety of experiments designed to relate complex formation to solution behavior, there is a lack of primary structural information to augment these data, information that may be necessary to adequately model the system energetics. Our recent application of high-energy x-ray scattering (HEXS) to aqueous solutions containing lanthanide or actinide ions is providing direct information about inner- and outer-sphere coordination environments. By coupling the scattering data with solvent extraction results used to determine equilibrium constants, we demonstrate the relative importance of short and longer-range correlations on the stability of dissolved ion complexes. This work has important implications for modeling efforts that seek a predictive understanding of metal ion speciation in solution. This work is supported by the DOE, OBES, Chemical Sciences, under contract DE-AC02-06CH11357.
11:30 AM - **Z6.2
Structural Characterization of Actinide(III)- and Lanthanide(III)-complexes With Partitioning Relevant N-donor Ligands.
Petra Panak 1 2 , Sascha Trumm 1 2 , Melissa Denecke 2 , Nidhu Banik 2 , Andreas Geist 2 , Thomas Fanghaenel 1 3
1 Department of Physical Chemistry, University of Heidelberg, Heidelberg Germany, 2 Institut für Nukleare Entsorgung, Karlsruher Institut für Technologie, Karlsruhe Germany, 3 Institute for Transuranium Elements, European Commission, Joint Research Centre, Karlsruhe Germany
Show AbstractPartitioning & transmutation (P&T) aims at reducing the long-term radiotoxicity of high-level nuclear wastes through separation of long-lived actinides from spent nuclear fuels and transmuting them by irradiation with neutrons into short-lived isotopes. The separation of trivalent actinides (An(III)), namely Am(III) and Cm(III), from lanthanides (Ln(III)) is a key step in the (P&T) strategy as some Ln nuclides have a high neutron absorption, which would reduce actinide transmutation efficiency. The chemical properties and the ionic radii of An(III) and Ln(III) are comparable, rendering high demands on the selectivity of the extraction ligand in liquid/liquid extraction processes. Various soft N-donor ligands have been developed, which separate An(III) from Ln(III) in liquid-liquid extraction with a high separation factor. Alkylated 2,6-di(1,2,4-triazin-3-yl)pyridines (BTP) and alkylated bis([1,2,4]triazin-3-yl)-[2,2’] bipyridinyls (BTBP) are such highly effective extraction ligands, which extract An(III) selectively over Ln(III) from nitric acid solutions into the organic phase with separation factors for Am(III) or Cm(III) vs. Eu(III) of > 100. Our research focuses on attempts to understand the underlying reason for the partitioning ligand selectivity which is not understood yet. Extended X-ray absorption fine structure (EXAFS) is used for determination of coordination structures for An(III) (U(III), Pu(III), Am(III), Cm(III)) and Ln(III) (Eu(III), Gd(III), Lu(III)) complexed with BTPs and BTBPs. Further spectroscopic actinide speciation by time-resolved laser fluorescence spectroscopy (TRLFS) provides information in the coordination sphere of the Ln(III)/An(III) complexes as well as on thermodynamic data of the complexation process. Combining the EXAFS and TRLFS results with our results from liquid-liquid extraction, we obtain fundamental information on the selectivity of N-donor ligands toward actinides over lanthanides on a molecular scale. These results can make a valuable contribution to a future design of highly efficient extraction ligands which can be used to optimize partitioning ligand design and, hence, extraction performance.
12:00 PM - Z6.3
Thermal Reactions of Nitrogen Trifluoride With Uranium and Plutonium.
Bruce McNamara 1 , Randall Scheele 1
1 , Pacific Northwest National Laboratory, Richland, Washington, United States
Show AbstractEarly studies of the reactions of uranium and the actinides with fluorinating reagents produced a wide range of actinide structures and effective new fluorinating reagents. Despite the hostile experimental environment that these compounds and reagents might create, a great deal of extraordinary research concerning uranium and actinide fluorination has been reported. USDOE funding for actinide fluorination research died away in the 1970’s. Consequently, experimental techniques that would have investigated the complicated surface chemistries associated with these reactions were not accessible and details of even the most simple fluorination systems remain unknown today. Investigation of the actinide fluorides promises access to new structures and reactivity, insights into 5f chemistries, access to new technologies and applications, and present still a high level of experimental challenge. Our studies focus on the use of a relatively newcomer reagent to thermal actinide fluorination studies, nitrogen trifluoride. This paper demonstrates the demanding nature of fluorination gas-solid reactions, which are in many ways analogous to oxidation or corrosion reactions, shows differences of the NF3 reactivity towards uranium and plutonium versus that of alternative fluorinating reagents, and re-opens the dialogue for fundamental studies in the fluorination of actinide materials.
12:15 PM - Z6.4
Syntheses and Characterizations of Neptunium Silicates.
Geng Bang Jin 1 , S. Skanthakumar 1 , L. Soderholm 1
1 Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois, United States
Show AbstractUranyl silicates have been extensively studied due to their environmental importance and structural diversity. The reported compounds involve hexavalent uranium and are all built upon the linear dioxo unit UO22+. Extension of the knowledge gained from these studies to the higher actinides is vitiated by the very different solution chemistry exhibited by the transuranics. Direct structural reports of Np silicates are very rare. Two hexavalent Np silicates (NpO2)2SiO4.2H2O (1) and K(NpO2)(SiO3OH).H2O (2) are known from powder X-ray diffraction analysis. They are isostructural with hexavalent uranium analogues. Very recently, syntheses and single-crystal structures of two pentavalent Np silicates Li6(NpO2)4(H2Si2O7)(HSiO4)2(H2O)4 and K3(NpO2)3(Si3O7) were reported.(3) Representing the predominant oxidation state in a aqueous solutions, these structures are different from the known expected uranyl chemistry under similar conditions. The nearly linear NpO2+ and UO22+ units have very similar coordination environments. However, NpO2+ units have a propensity to form cation-cation interactions in which the oxo moiety on one NpO2+ unit also serves as an equatorial ligand on a neighboring neptunyl(V) unit. Neptunium silicates are expected to deviate significantly from the structural chemistry of the known uranium silicates.In our laboratory, we are interested in studying structural motifs adopted by transuranic actinyl silicates. Here we report the synthesis and structural characterization of a new neptunium silicate. Na2Np(V)O2SiO3OH was synthesized by treating the mixture of NpO2+ stock solution, SiO2 powder, Na2SiO3 and NaOH solutions hydrothermally at 180 degree for two weeks. Na2NpO2SiO3OH crystallizes in the space group Pbcn with a different structural topology from other known Np silicates. The structure of Na2NpO2SiO3OH consists of two-dimensional NpO2SiO3OH- layers separated by Na+ cations. Within the NpO2SiO3OH- layers, each neptunyl pentagonal bipyramid shares equatorial edges with each other along the c axis to form a single chain, that further connected by tetrahedral silicate group along the b axis. The magnetic properties of neptunium silicates are studied to investigate how the lattice connectivity influences the extended electronic structure.Work at ANL was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences, under contract DE-AC02-06CH11357.References(1)Bessonov, A. A.; Grigoriev, M. S.; Ioussov, A. B.; Budantseva, N. A.; Fedosseev, A. M. Radiochim. Acta 2003, 91, 339.(2)Andreev, G. B.; Fedosseev, A. M.; Perminov, V. P.; Budantseva, N. A. Radiokhimiya 2003, 45, 438.(3)Forbes, T. Z.; Burns, P. C. Inorg. Chem. 2008, 47, 705.
12:30 PM - **Z6.5
Hybrid Materials From the f- Elements: Hydrothermal Syntheses, Crystal Chemistry and Spectroscopy.
Christopher Cahill 1 , Kara Knope 1 , Nicholas Deifel 1 , Clare Rowland 1 , Richard Wilson 2
1 Department of Chemistry, The George Washington University, Washington, District of Columbia, United States, 2 Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois, United States
Show AbstractUnderstanding the interaction of the uranyl (or neptunyl) cation with small organic molecules such as carboxylates and phosphonates is of critical importance during several stages of the nuclear fuel cycle. While much is known about these interactions in the solution phase, the structural chemistry remains less developed. Our efforts over the past several years have been to produce solid-state compounds containing these species and to systematically explore their crystal chemical features. Our philosophy has been consistent with much of the efforts in the transition metal community to produce the popular Metal-Organic Framework (MOF) materials: assemble metal centers through multi-functional organic linker species to produce higher dimensional solids. The local geometry of the various metal centers then provides insight to coordination chemistry preferences of relevant functional groups in the solid state. Presented will be an overview of various uranyl and neptunyl carboxylates and phosphonates produced using this approach. Details on actinyl hydrolysis and in situ ligand syntheses will be provided as well as these phenomena contribute uniquely to structural diversity in these systems.
Z7/AA5: Joint Session: Actinide Chemistry
Session Chairs
Thomas Fanghaenel
Lynne Soderholm
Blas Uberuaga
Wednesday PM, April 07, 2010
Room 3008 (Moscone West)
2:45 PM - **Z7.1/*AA5.1
High-resolution 17O NMR Nuclear Magnetic Resonance Studies of Uranium Oxides: Preliminary Results.
Ian Farnan 1 , Kevin Boland 2 , David Clark 3
1 Earth Sciences, University of Cambridge, Cambridge United Kingdom, 2 Inorganic, Isotope, and Actinide Chemistry (C-IIAC, Los Alamos National Laboratory, Los Alamos, New Mexico, United States, 3 Seaborg Institute, Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Show AbstractNuclear magnetic resonance is an element specific technique that can provide unique quantitative information about atomic distributions amongst different sites in a material. In the case of actinides, the large onsite hyperfine coupling of 5f electrons to the nucleus means that nuclear spin relaxation lifetime effects will make direct observation of the actinide nucleus extremely difficult. However, observation of resolved transferred hyperfine shifts at adjacent nuclei may well be an extremely powerful probe of local sites and their occupation. We have applied magic-angle spinning NMR to U17O2 to determine the resolution possible as a proof of principle experiment. We have obtained a static NMR spectrum of U17O2 at 9.4 Tesla with a width of 42 kHz (FWHM) in agreement with literature values for static spectra. Subsequent MASNMR spectra of the same sample with increasing spinning speeds of 5, 10, 15 kHz show that the broad line shape breaks up into a central band and sidebands such that resolution increases with increased spinning speed. A minimum centreband linewidth at 15 kHz spinning of 3.2 kHz is obtained. This represents a width of 60 ppm in terms of local field, thus resolution of transferred hyperfine shifts differing by 6-10 ppm should be possible. The total NMR shift observed for U17O2 is 726 ppm from H2O. Temperature dependent measurements of the shift indicate that the Fermi contact contribution is ~ 160 ppm. This indicates very little delocalisation of the U4+ 5f2 unpaired electron density to the nearby (2.35 Å) oxygen atoms. A preliminary spectrum of U4O9, which will contain adventitious oxygens, is less well-resolved at 15 kHz spinning, but indicates the presence of more than one site. Ongoing NMR resolution enhancement protocols and/or faster spinning and lower magnetic fields should make resolution and identification of oxygen sites in this material a tractable problem.
3:15 PM - Z7.3/AA5.3
Radiation Range and Damage Assessment in UO2 Simulated byClassical Molecular Dynamics.
Byoungseon Jeon 1 , Anurag Chaudhry 1 , Mark Asta 2 , Steve Valone 3 , Niels Gronbech-Jensen 1
1 Dept. of Applied Science, University of California, Davis, Davis, California, United States, 2 Dept. of materials science and engineering, University of California, Berkeley, Berkeley, California, United States, 3 Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Show AbstractMolecular Dynamics (MD) has been used extensively to study the crystal damage production and short time evolution in UO2 due to Primary Knock-on Atoms (PKAs). We here present an approach based on a combination of MD strategies. First, to characterize the radiation range, REED-MD [1,2] and binary collision methods [3] are used and compared with experiments on single/poly crystalline UO2. Contributions to the atomic force fields are nuclear-nuclear, electron muffin-tin drag forces, and electron stopping. The effect of the target material structure and channeling is discussed. Secondly, full massively parallel MD cascade simulations have been done to evaluate the damage on UO2 matrix, yielding displacement cascades with. Confirming the migration of defects and recovery of matrix, temporal variation of energy landscape are shown. Through extensive analysis, the behavior of defect and damage evolution will be addressed.[1] K. M. Beardmore, N. Gronbech-Jensen, Physical Review E, v.57, pp.7278-7287, 1998[2] B. Jeon and N. Gronbech-Jensen, Computer Physics Communications, v.180, pp.231-237, 2009[3] www.srim.org
3:30 PM - Z7.4/AA5.4
Electronic Structure and Ionicity of Actinide Oxides from First Principles.
Leon Petit 1 2 , Axel Svane 2 , Walter Temmerman 1 , Zdzislawa Szotek 2 , George Stocks 3
1 Computational Science and Engineering Department, Daresbury Laboratory, Warrington United Kingdom, 2 Department of Physics and Astronomy, Aarhus University, Aarhus Denmark, 3 Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show AbstractThe ground state electronic structures of the actinide oxides AO, A2O3 and AO2 (A=U, Np, Pu, Am, Cm, Bk, Cf) are determined from first-principles calculations, using the self-interaction corrected local spin-density (SIC-LSD) approximation. Emphasis is put on the degree of f-electron localization, which for AO2 and A2O3is found to follow the stoichiometry, namely corresponding to A4+ ions in the dioxide and A3+ ions in the sesquioxides. In contrast, the A2+ ionic configuration is not favorable in the monoxides, which therefore become metallic. The energetics of the oxidation and reduction of the actinide dioxides is discussed, and it is found that the dioxide is the most stable oxide for the actinides from Np onwards. Our study reveals a strong link between preferred oxidation numberand degree of localization which is confirmed by comparing to the ground state configurations of the correspondinglanthanide oxides. The ionic nature of the actinide oxides emerges from the fact that only those compounds will form where the calculated ground state valency agrees with the nominal valency expected from a simple charge counting.
4:15 PM - **Z7.5/*AA5.5
Actinide Solid/Solution Interface Chemistry Relevant to Nuclear Waste Disposal.
Horst Geckeis 1
1 Institute of Nuclear Waste Disposal, Karlsruhe Institute of Technology, Karlsruhe Germany
Show AbstractAssessment of environmental actinide behaviour in the environment requires fundamental insight into molecular structures of relevant actinide species. Recently, we investigated in detail colloid generation, solid/liquid interface reactions [1] and solid-solution formation of various actinides [2]. A sound understanding of such processes is required in order to allow a reliable prediction on actinide mobility or retention under nuclear waste repository conditions and in contaminated sites. Spectroscopic and classical batch type experiments and quantum chemistry calculations have been applied to obtain a consistent picture on actinide speciation and structures on mineral and colloid surfaces upon outer-sphere sorption and inner-sphere surface complex formation. Beside surface phenomena, incorporation into mineral structures appears to be a common reaction for actinides with minerals. Trivalent actinides and lanthanides have been taken as fluorescent probes to study incorporation reaction mechanisms and structural features of incorporated actinide ions. Solid-solution formation with the exchange of Ca ions vs. actinide ion has been verified as a relevant reaction with calcium carbonates (calcite, aragonite and vaterite) and apatites. Under certain conditions, however, actinides may also be integrated into hydroxides [3] and aluminosilicates. [1] H. Geckeis, Th. Rabung, J. Cont. Hydrol., 102, 2008, 187-195[2] M. Schmidt et al., Dalton Trans., 2009, 6645 – 6650[3] N. Huittinen et al., J. Coll. Interface Sci., 332, 2009, 158-164
4:45 PM - Z7.6/AA5.6
Characterization of the Penetration Mechanisms of Water into Polycrystalline UO2.
Ilaria Marchetti 1 , Fabio Belloni 1 , Paul Carbol 1 , Jerome Himbert 1 , Thomas Fanghaenel 1
1 Institute for Transuranium Elements, European Commission - Joint Research Centre, Eggenstein-Leopoldshafen Germany
Show AbstractIn the event of exposure of spent nuclear fuel to groundwater in a final repository, the mobilization of radionuclides will be affected by the modes of water attack. In particular, possible mechanisms of preferential dissolution of grain boundaries rather than matrix dissolution would cause a rapid increase of the surface area exposed to groundwater, with effects on the fraction of inventory becoming available for prompt dissolution and on the overall mechanical stability of the spent fuel. We conducted static corrosion experiments with 18O-labelled water on polycrystalline UO2 at room temperature, under monitored pH and Eh conditions. Analysis of the sample matrix was carried out by means of SEM, SIMS and high-resolution profilometry, while solution analysis for the measurement of the dissolution rate of uranium was performed by ICP-MS. SIMS depth profiling on the leached pellet showed two diffusion regimes. First, shallow depth profiling up to a depth of a few tens nm showed a short-range diffusion of 18O at high concentration (in the order of ten percent) which is compatible with Fick's lattice diffusion regime. Then, a smaller deviation from the natural 18O/(16O + 18O) isotopic ratio was measured up to a depth of 20 µm, revealing a long-range, low-concentration (in the order of a few permille, raster-averaged) diffusion regime that can be attributed to the penetration of water through grain boundaries, behaving as “high-diffusivity paths”. Fisher's, Whipple's and Levine-MacCallum's models have been used to fit the long-range experimental profiles and derive a first estimate for the grain-boundary diffusion coefficient, while the lattice diffusion coefficient has been retrieved by fitting the short-range profiles with a solution of Fick's law. Our results prove to be quite realistic, in spite of their divergence from those previously reported by other authors, whose experimental approaches however involved much higher temperatures and less direct measurement techniques. In this respect, SIMS is possibly the most powerful tool for this sort of application, as it can guarantee a direct observation of both 18O short-range diffusion – with a nanometre resolution – and water diffusion at large penetration depths. This kind of studies shows the potentiality to provide an overall frame of the corrosion/diffusion phenomena involved in the water attack on UO2, and to be extended to other polycrystalline wasteforms as well.
5:00 PM - **Z7.7/AA5.7
Nano-scale Actinide-based Clusters.
Peter Burns 1
1 Department of Civil Engineering and Geological Sciences, University of Notre Dame, Notre Dame, Indiana, United States
Show AbstractThis presentation will emphasize our current research concerning actinide-based nano-scale clusters. New clusters that will be examined include those containing pyrophosphate and oxalate ligands (more than a dozen new structures.) Emphasis will include fullerene topologies of uranyl polyhedra.
5:30 PM - Z7.8/AA5.8
Conjugates of Magnetic Nanoparticle-Actinide Chelator for Used Fuel Separation.
You Qiang 1 , Maninder Kaur 1 , Andrew Johnson 2 , Hongmei Han 1 , Jozef Kaczor 2 , Andrzej Paszczynski 2
1 1Department of Physics and Environmental Science Program, University of Idaho, Mscow, Idaho, United States, 2 Environmental Biotechnology Institute, University of Idaho, Mscow, Idaho, United States
Show AbstractThere is a significant achievement recently on nuclear fuel recycle technology based on utilizing conjugates of magnetic nanoparticle-chelator (MNP-Che) to separate the acidic nuclear aqueous waste.1-3 Based on the literature review of the progress on this magnetic separation technology, we have chosen the environmentally benign oxa-diamide chelator4 to conjugate with the core-shell MNPs for nuclear waste separation, which has the potential to make the separation process more efficient than the traditional processes using organophosphorus chelators. The oxa-diamide chelator was coupled to MNPs by reaction of an acyl chloride group on the terminal end of activated oxa-diamide with primary amines introduced on the surface of the MNPs to form a stable amide bond between the chelators and MNPs. The key issues for scaling up this application are the loading density of chelators onto the MNPs for an efficient sorption and the stability of the coated MNPs under harsh process conditions (such as highly acidic). To address these issues, different coatings and reaction chemistries were used to increase the chelator loading capacity and the MNP-Che's stability. Infrared and mass spectrometers were used to study the stability of the conjugated complex. Morphologies of the conjugated complex were characterized by transmission electron microscope; magnetic properties of MNPs and coated MNPs as well as MNP-Che complex were characterized by vibrating sample microscope. We found that the polyamine used for the conjugation process dramatically increased the density of amine groups on the MNPs, which is beneficial for the actinide sorption. The silica coating for the MNPs before attachment of chelators improves chelator loading on the MNPs by providing stable attachment surface and increasing density of hydroxyl groups, which facilitates its application in the nuclear aqueous waste separation. 1. L. Nuñez, B. A. Buchholz, M. Kaminski, S. B. Aase, N. R. Brown, G. F. Vandegrift. Separation Science and Technology 31, 1393 (1996).2. C. Gruttner, V. Bohmer, A. Casnati, J. Dozol, D.N. Reinhoudt, M. M. Reinoso-Garciae, S. Rudershausena, J. Teller, R. Ungaroc, W. Verboome, P. Wang. JMMM 293, 559-566 (2005).3. R. D. Ambashta, P. K.Wattal, S. Singh, & D. Bahadur. Separation Science and Technology 41, 925-942 (2006).4. G. X. Tian, L. F. Rao, S. J. Teat, and G. K. Liu. Chemistry-a European Journal 15, 4172 (2009).
Symposium Organizers
Scott McCall Lawrence Livermore National Laboratory
Eric Bauer Los Alamos National Laboratory
John Gibson Lawrence Berkeley National Laboratory
Thomas Fanghaenel European Commission Joint Research Center
Lynda Soderholm Argonne National Laboratory
Z8: Actinide Metallurgy
Session Chairs
Kerri Blobaum
Ladislav Havela
Thursday AM, April 08, 2010
Room 3008 (Moscone West)
9:15 AM - **Z8.1
On the Potential for Vacancy Annihilation as a Mechanism for Conditioning in Pu-1.9 at.% Ga.
Jason Jeffries 1 , Kerri J. M. Blobaum 1 , Adam Schwartz 1
1 , Lawrence Livermore National Laboratory, Livermore, California, United States
Show AbstractThe δ→α’ martensitic transformation in Pu-1.9 at.\% Ga occurs when the alloy is cooled below about -100 °C. This transformation is isothermal and exhibits a remarkable susceptibility to an ambient-temperature isothermal hold (referred to as conditioning) prior to the transformation. This “conditioning” effect can dramatically increase the amount of transformation that occurs at low temperature; however, the mechanism by which conditioning affects the δ→α’ transformation is not known. This conditioning effect may be a more general phenomenon, and thus knowledge of the mechanism responsible for conditioning is of great importance to understanding the δ→α’ transformation itself as well as the general circumstances that can affect martensitic phase transformations. Using differential scanning calorimetry measurements, vacancy annihilation as a mechanism for the conditioning effect has been examined. While our findings reveal some characteristics of the conditioning effect that are reminiscent of vacancy annihilation, we show that these results imply that vacancy annihilation is not a likely candidate description for the conditioning effect.Lawrence Livermore National Laboratory is operated by Lawrence Livermore National Security, LLC, for the US Department of Energy, National Nuclear Security Administrations under Contract DE-AC52-07NA27344.
9:45 AM - Z8.2
In situ X-ray Diffraction Investigation of the Delta to Alpha-prime Transformation in Pu-Ga Alloys.
Kerri Blobaum 1 , Jason Jeffries 1 , Mark Wall 1 , Adam Schwartz 1
1 , Lawrence Livermore National Laboratory, Livermore, California, United States
Show AbstractThe time-temperature-transformation (TTT) curve for the delta to alpha-prime isothermal martensitic transformation in a Pu-1.9 at. % Ga alloy is peculiar because it is reported to have a double-C shape. Recent work suggests that an ambient temperature conditioning treatment enables the lower-C curve. However, the mechanisms responsible for the double C are still not fully understood. When the delta to alpha-prime transformation is induced by pressure, an intermediate gamma-prime phase is observed in some alloys. It has been suggested that transformation at upper-C temperatures may proceed via this intermediate phase, while lower-C transformation progresses directly from delta to alpha-prime. To investigate the possibility of thermally induced transformation via the intermediate gamma-prime phase, in situ x-ray diffraction at the Advanced Photon Source was performed. Using transmission x-ray diffraction, delta to alpha-prime transformation was observed in samples as thin as 30 µm as a function of time and temperature. The intermediate gamma-prime phase was not observed at -120°C (upper-C curve) or -155°C (lower-C curve). Results indicate that transformation was initially rapid upon reaching -120°C, but the rate slowed within minutes. Transformation at -155°C was more rapid. Differences between the kinetics and mechanisms of the upper- and lower-C will be discussed.Lawrence Livermore National Laboratory is operated by Lawrence Livermore National Security, LLC, for the US Department of Energy, National Nuclear Security Administrations under Contract DE-AC52-07NA27344.
10:00 AM - Z8.3
Thermodynamic Characteristics of Phase Transformations in Plutonium.
Franz Freibert 1 , Tarik Saleh 1 , Jeremy Mitchell 1 , Dan Schwartz 1
1 Nuclear Materials Science Group, Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Show AbstractAt ambient pressures, plutonium is the element with the greatest number of allotropic phases. These transformations are marked by extremely large volumetric changes, which arguably lead to substantial thermal hysteresis and incomplete phase reversion on cooling. Low symmetry phases (α, β, and γ) give way to a higher symmetry phase (δ) with as little as 1 Ga atom per 50 Pu atoms substituted into the lattice. The phase stabilization effects of Ga are not well understood and are being explored with the introduction of soluble concentrations into otherwise unalloyed plutonium. The impact of this chemical impurity on phase transformations and kinetics will be explored through changes in measured reversible and irreversible thermodynamic properties and discussed in light of a traditional understanding of phase transformations.
10:15 AM - Z8.4
Resonant Ultrasound Spectroscopy Studies of Transient Phenomena in Plutonium Alloys.
Tarik Saleh 1 , Jeremy Mitchell 1 , Kerri Blobaum 2 , Franz Freibert 1
1 Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, New Mexico, United States, 2 Condensed Matter and Materials Division, Livermore National Laboratory, Livermore, California, United States
Show AbstractRoom temperature stabilized face centered cubic delta plutonium alloys undergo a martensitic transformation to an expanded monoclinic alpha prime phase during cooling below room temperature. Subsequent heating and cooling cycles reveal both hysteresis in the δ-α’ transformation as well as different behavior during subsequent cycling. Resonant ultrasound spectroscopy (RUS) measurements of elastic moduli were performed between thermal cycles to look for transient microstructural changes during the initial stages of room temperature aging. This experiment was designed to test for the conditioning effect mentioned in literature studies. Results from these studies will be presented along with RUS results from other plutonium alloys with coexistent phases.
10:30 AM - Z8.5
Burst Martensitic Transformations in the Pu System.
Daniel Schwartz 1
1 MST-16, Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Show AbstractPure plutonium and Pu-2 at. % Ga both exhibit characteristic burst martensitic transformations. The transformations occur at significantly different temperatures and are in several ways unlike conventional burst martensitic transformations in other systems (e.g. Fe-Ni-C). The two burst transformations in the Pu system will be compared and contrasted to elucidate the breadth of behavior seen for this class of transformation.Pu-2 at. % Ga can be stabilized as the fcc delta phase at room temperature. On cooling below -100C, up to ~25% of the delta phase transforms to the monoclinic alpha prime phase. When the material is heated, this alpha prime reverts to delta at ~50C in a burst transformation. In pure Pu, the burst transformation occurs when delta phase material is cooled to the gamma phase region at ~280C. Both of these transformations occur at homologous temperatures that are far above the temperatures where burst transformations typically occur in other systems. The alpha prime-delta transformation is characterized by near-periodic bursts as the material is heated, while the delta-gamma burst exhibits no periodic character. The envelop of the alpha prime-delta burst event is symmetric, as opposed to a sharp onset followed by a gradual and extended decline for the delta-gamma transformation. In both cases, a large volume difference exists between the phases: 24% difference between alpha prime and delta and 8% difference between delta and gamma. However, in the alpha prime-delta case the phase with the larger volume (delta) is the final state, while the phase with the smaller volume (gamma) is the end state for the delta-gamma transformation.Thermodynamic arguments using a Clausius-Clapeyron analysis will be presented to account for a burst-arrest scenario, based on pressure changes due to the formation of new phase. The asymmetrical nature of these transformations, i.e. their different character on cooling and heating through the transformation temperature, cannot be directly explained thermodynamically. The asymmetry is likely to be due to microstructural and lattice defect effects, and this argument will be presented in detail.
10:45 AM - Z8: ActaMet
BREAK
11:15 AM - Z8.6
Elastic Moduli of Pure Alpha, Beta, Gamma Plutonium— Three Different Metals.
Albert Migliori 1 , Yoko Suzuki 1 , Victor Fanelli 1 , Jon Betts 1 , Izabel Stroe 1 2 , Franz Freibert 1 , Jeremy Mitchell 1
1 MPA-CMMS, LANL, Los Alamos, New Mexico, United States, 2 Physics, Worcester Polytechnic Institute, Worcester, Massachusetts, United States
Show AbstractFrom 10 K to 580K plutonium changes phase from monoclinic alpha to body centered monoclinic beta to orthorhombic gamma structures. Each crystal structure is rare or unique for an elemental metal. Measurements presented here provide the first high-accuracy values for a single high-purity specimen of the elastic moduli of unalloyed polycrystal plutonium as a function of temperature throughout the entire range of existence of the alpha, beta, and gamma phases. The bulk and shear moduli, essential thermodynamic material properties, reflect important and huge changes with temperature, such that these phases present as three different metals. Unlike phase transformations in many other elements where the bonding, nearest-neighbor distances, and physical properties are closely related among phases, in the three lowest-temperature phases of plutonium, the relationships are missing, and support the extreme sensitivity of plutonium properties to phase, temperature, and almost-certainly, electronic structure. We describe here the characteristics and implications of these newly-observed properties.
11:30 AM - Z8.7
Effects of Age-related Defects on the Thermophysical Properties of Plutonium.
Jeremy Mitchell 1 , Franz Freibert 1 , Tarik Saleh 1 , Adam Farrow 1 , Daniel Schwartz 1
1 Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Show AbstractPlutonium is an extraordinarily complex metal and the effect of aging further complicates our understanding of the properties of plutonium and its alloys. We will describe our ongoing research on the phase stability of young and aged Ga-stabilized delta-phase plutonium using dilatometry, differential scanning calorimetry, resonant ultrasound spectroscopy, and immersion density. Our discussion will focus on experimental results on (1) the low-temperature transformation of fcc delta Pu to monoclinic alpha prime and (2) annealing behavior above room temperature. Thermal cycling experiments show that the as-aged, first-cycle properties differ from the subsequent cycles. Both types of experiments show that a particular defect structure influences as-aged properties; once these materials are heated to above 150 °C, this defect is annealed and the material exhibits behavior similar to that of a young sample.
11:45 AM - Z8.8
Ab initio Elastic Properties for Pu Metal and Pu-Ga Alloy.
Per Soderlind 1 , Alex Landa 1 , John Klepeis 1
1 , Lawrence Livermore National Laboratory, Livermore, California, United States
Show AbstractWe employ density-functional theory (DFT) to calculate elastic properties ofall known ambient-pressure phases of Pu metal and for the face-centered-cubic Pu-Ga alloy. For α, β, γ, δ, and δ' plutonium the elastic properties compare reasonably with resonant-ultrasound spectroscopy for polycrystal Pu. Prior to melting Pu crystallizes in a body-centered-cubic phase (ε) while calculations predict this phase to be mechanically unstable (tetragonal shear constant is negative). This inconsistency between theory and phase diagram occurs also for the earlier actinides as well as some d-transition metals (Group IV for example) and is believed to be due to anharmonic temperature effects not related to f-electron correlation. The Pu-Ga alloy system is studied for Ga content up to 10% by sophisticated alloy-DFT techniques. Ambient equation-of-state and elastic properties are consistent with measured data and show softening with increasing Ga concentration.This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract No. DE-AC52-07NA27344.
12:00 PM - **Z8.9
Capturing the Double-well Potential in Pu.
Carlos Arguello 1 , Chris Marianetti 1
1 , Columbia University, New York, New York, United States
Show AbstractElemental Pu displays many anomalous properties which continue to challenge our computational approaches and our imaginations. What is missing is a minimalistic model which captures the essence of this material, and allows one to start from a minimalistic approximation. In particular, we focus on understanding the double-well potential in Pu. We demonstrate that the periodic Anderson model is a minimalistic model of Pu, and that solving it within Hartree-Fock gives rise to the double-well potential. The Hartree-Fock solution is compared to progressively more complex approximations, such as the dynamical mean-field theory, elucidating the role of static vs. dynamic correlations. Implications for various properties and phenomena will be addressed.
12:30 PM - Z8.10
Evolution of Static Physical Properties in Plutonium by Self-irradiation Damage.
Brandon Chung 1 , Kenneth Lema 1 , David Hiromoto 1
1 , Lawrence Livermore National Laboratory, Livermore, California, United States
Show AbstractOver the last several years, we have demonstrated the utility of immersion density, dilatometry and static mechanical tests for determining the aged-related change in physical properties of plutonium alloys. Immersion density and dilatometry measurements continue to show decreasing density and expanding volume with age. These changes appear to be dominated by helium in-growth at a rate of 0.002% per year in density and volume. Aging also causes increasing yield and ultimate tensile strengths followed by possible saturation past 70 equivalent years of age. We have identified two age-related phenomena due to self-irradiation in plutonium alloys that cause observed changes in physical properties: the initial transient from the initial cascade damage and helium bubble accumulation. The third possible age-related phenomenon is void swelling, but this has not yet been observed. Although we have measured many peculiar properties of plutonium alloys, many outstanding problems remain and new issues regularly appear. In this paper, we will discuss unresolved problems in measured density, dilatometry and static strength and our approaches to elucidating some of these questions will be addressed. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.
12:45 PM - Z8.11
Aging Mechanisms in U-Nb Alloys- A Neutron Pair Distribution Function Analysis.
Alice Smith (Acatrinei) 1 2 , Heather Volz 2 , Robert Hackenberg 2 , Thomas Proffen 3 , Robert Field 2 , Tim Tucker 2
1 MST-16: Nuclear Materials Science, Los Alamos National Laboratory, Los Alamos, New Mexico, United States, 2 MST-6:Materials Technology-Metallurgy, Los Alamos National Laboratory, Los Alamos, New Mexico, United States, 3 LANSCE-LC, Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Show AbstractUranium and its alloys have multiple applications based on their high densities and nuclear properties. Uranium alloys show improved corrosion resistance and mechanical properties, such as density, strength and ductility. Aging reactions influence the elastic and plastic stress/strain response.This study utilized depleted uranium alloys containing 5.6 and 7.7 Nb (wt%). Samples were artificially aged by isothermal treatments at 373, 473, 523 and 573K for times varying from 10 to 100,000 minutes, resulting in significant age hardening. We present a neutron pair-distribution function analysis of the aged U-Nb alloys, in order to better understand and explain the aging mechanism and the induced changes. Our measurements were collected at 100K on the Neutron Powder Diffractometer (nPDF) at the Manuel Lujan Jr. Neutron Scattering Center, LANSCE, Los Alamos National Laboratory. The instrument is designed for pair-distribution function studies of disordered and nanocrystalline materials, and equally well-suited for high resolution crystallographic studies. Previous Rietveld refinements from laboratory X-ray powder diffraction measurements show a time dependent lattice parameters reversal in the case of the U-5.6Nb specimens, and a decrease in unit cell volumes for the U-7.7Nb specimens [1].[1]. H.M.Volz et al, Journal of Alloys and Compounds, 444-445 (2007) 217-225
Z9: Electronic Structure
Session Chairs
Thursday PM, April 08, 2010
Room 3008 (Moscone West)
2:30 PM - **Z9.1
Electronic Properties of ζ-U-Pu Alloys.
Ladislav Havela 1 , Anna Adamska 1 , Rachel Eloirdi 2 , Eric Colineau 2 , Jean-Christophe Griveau 2 , Daniel Bouexiere 2 , Alexander Shick 3
1 Department of Condensed Matter Physics, Charles Universtity, Prague 2 Czech Republic, 2 European Commission, Joint Research Centre, Institute for Transuranium Elements, Karlsruhe Germany, 3 Institute of Physics, Academy of Sciences of the Czech Republic, Prague 8 Czech Republic
Show AbstractTwo phases exist in the U-Pu phase diagram. About 10% Pu can be dissolved in orthorhombic α-U. In addition, a mysterious ζ-U-Pu phase exists over a certain composition range (35-70 % U) around the middle of the U-Pu phase diagram. So far only structure data exist for these phases. The cubic (rhombohedrally distorted) ζ-U-Pu phase, the structure details of which have not been resolved for a long time, has 10 different crystallographic positions, randomly occupied by U and Pu [1]. Mapping it onto the systematics of Pu allotropic phases, its volume fits about half way between α- and β-Pu. The volume density of 18.55 g/cm3 should be compared with 17.70 g/cm3 for β-Pu and 19.92 g/cm3 for α-Pu. So as to provide a basic characterization of electronic properties, we performed a study of magnetic properties and specific heat for the samples with concentrations U59Pu41 and U41Pu59 synthesized by arc melting and annealing. Magnetic susceptibility measurements revelaed that both compounds are weak paramagnets, with susceptibilities very weakly increasing from the room temperature values reaching 8.5*10-9m3/mol and 7.7*10-9 m3/mol for the Pu and U rich sample, respectively, which is higher than the α-U value 4.8*10-9m3/mol or 6.7*10-9m3/mol for α-Pu. Also the respective γ-values 23 mJ/mol K2 for U0.59Pu0.41 and 35 mJ/mol K2 for U0.41Pu0.59 are higher than for the α-phases of both metals. A tentative analysis of the Pu contribution will be presented and experimental data will be confronted with results of state-of-the-art electronic structure calculations.[1] A.C. Lawson et al., Acta Cryst. B52 (1996) 32.
3:00 PM - Z9.2
Electronic Structure Theory of PuAm and PuCe Alloys.
Jindrich Kolorenc 1 2 , Alexander Shick 1 , Ladislav Havela 3 , Alexander Lichtenstein 2
1 , Institute of Physics ASCR, Prague Czech Republic, 2 , University of Hamburg, Hamburg Germany, 3 , Charles University, Prague Czech Republic
Show AbstractElectronic and spectroscopic properties of heavy actinides attractedrecently considerable attention. In this work, we study theoreticallythe effects of electron correlations on the electronic structure andspectra of heavy actinides Pu, Am, and their alloys. We focus oncomparison between the theory and available experimental results forvalence photoelectron spectra (PES) as well as for XAS/EELS. We makeuse of the LDA+Hubbard I approximation implemented in thefull-potential LAPW basis, including self-consistency over the chargedensity [1]. Our approach is all-electron, includes spin-orbitinteraction and makes no shape approximations for the charge density.Our results for PES are in good agreement with experimental data andwith previous LDA+DMFT calculations [2]. Also, we find good agreementwith branching ratios measured in XAS/EELS [3]. Analysis of the J=5/2and J=7/2 contributions to the f-occupation supports theintermediate-coupling picture of f-states in heavy actinides and theiralloys. The electronic specific heat coefficient is calculated forPuAm and PuCe alloys in reasonable agreement with recent experimentaldata. We show that Pu-atoms in PuAm and PuCe alloys keep theirintermediate-valence character.[1] A. B. Shick, J. Kolorenc, L. Havela, and A. I. Lichtenstein, Phys. Rev. B 80, 085106 (2009).[2] J. H. Shim, K. Haule, S. Savrasov, and G. Kotliar, Phys. Rev. Lett. 101, 126403 (2008).[3] K. Moore and G. van der Laan, Rev. Mod. Phys. 81 235, (2009).
3:15 PM - **Z9.3
Electronic and Magnetic Properties of the Uranium and Plutonium Dipnictides.
Jian-Xin Zhu 1 , M. Jones 2 , A. McMahan 3 , R. Albers 1 , J. Wills 1
1 , Los Alamos National Laboratory, Los Alamos, New Mexico, United States, 2 , University at Buffalo, SUNY, Buffalo, New York, United States, 3 , Lawrence Livermore National Laboratory, Livermore, California, United States
Show AbstractFirst-principles electronic structure calculations have been performed for uranium and plutonium dipnictides based on the density functional theory. The magnetism in these compounds is investigated with only spin-polarization and also with the orbital polarization correction. It is found that the inclusion of orbital polarization correction improves the comparison between the theory and experiments. The Fermi surface topology and the determination of extremal orbitals are presented. The correlation effects are also evaluated in these compounds.
3:45 PM - Z9.4
5f Electronic Structure and Fermiology of Pu Materials.
J. Joyce 1 , T. Durakiewicz 1 , K. Graham 1 , E. Bauer 1 , D. Moore 1 , J. Mitchell 1 , J. Kennison 1
1 , Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Show AbstractThe electronic structure of PuCoGa5 and PuSb2 is investigated using angle-resolved photoemission (ARPES) and compared to results for Pu metal. Details of the sharp quasiparticle peak at the Fermi energy are presented, including temperature dependence, giving insight into the mechanisms which give rise to strongly correlated characteristics in these materials. For PuSb2, the high resolution ARPES data indicate a quasiparticle peak that is no more than 80 meV wide and centered mainly below the Fermi energy. Additionally, the details of the emergent character of the 5f electrons are explored via characterization of more localized 5f states well removed from the Fermi energy. The new Pu ARPES capability has demonstrated 3 meV energy resolution with 0.2 degree angular resolution and a 10 to 350 K temperature range allowing a wide range of new experiments in Pu electronic structure. Temperature dependent photoemission provides insight into the electron-phonon coupling for these Pu materials and is consistent with the modest enhanced mass observed for these Pu systems. Both PuSb2 and PuCoGa5 ARPES data show a dispersive peak which crosses the Fermi energy at a photon energy of 21.2 eV where the conduction states have a larger cross section than the 5f states. The results for 40.8 eV photon energy, with enhanced 5f strength, indicate a peak dispersing through the Fermi energy for PuCoGa5 but the PuSb2 data indicate a non-dispersive peak near but not at the Fermi energy. The photon energy and temperature dependence of the Pu ARPES data will be discussed in terms of 5f emergent phenomena.Work supported by the U.S. Department of Energy, Basic Energy Sciences, the Los Alamos National Laboratory LDRD program, and Campaign II.
4:00 PM - Z9: Elect Stru
BREAK
Z10: Physical Properties of Actinides
Session Chairs
Thursday PM, April 08, 2010
Room 3008 (Moscone West)
4:30 PM - **Z10.1
Quantum Criticality in Uranium Systems as a Function of Size.
Greg Stewart 1 , Jungsoo Kim 1
1 Physics, University of Florida, Gainesville, Florida, United States
Show AbstractRecently [1] a study of specific heat, C, and magnetic susceptibility, χ, at low temperatures, T, of quantum critical Ce(Ru0.4Rh0.6)2Si2 as a function of size down to ~ 1 μ was carried out. The goal was to study the divergent C/T and χ down into a regime where the sample size might be smaller than the spatial extent of the fluctuations. In that work it was found that as the material was made smaller unavoidable defects from the grinding process caused local magnetic moments in the Ce sublattice, presumably due to an interruption of the Kondo screening process. These moments caused the physical properties to be dominated by rare magnetic clusters, or 'Griffiths phases.' The present work repeats this experiment in a uranium quantum critical system. The goal is to determine the effect of size on the very long range fluctuations present at a quantum critical point, without the extrinsic (and dominating) effect of Griffiths phases. Uranium systems should be less susceptible than Ce systems to defect-induced magnetic moments.[1] J. S. Kim, et al., Phys. Rev. B79, 165119 (2009).
5:00 PM - Z10.2
An Investigation into Possible Changes in the Structure of the Heavy Fermion UBe13 Due to Growth Method.
Heather Volz 1 , James Smith 1 , Sven Vogel 2 , Alice Acatrinei 3
1 MST-6: Materials Technology - Metallurgy, Los Alamos National Laboratory, Los Alamos, New Mexico, United States, 2 LANSCE- Lujan Center, Los Alamos National Laboratory, Los Alamos, New Mexico, United States, 3 MST-16: Nuclear Materials Science, Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Show AbstractAs with many actinide compounds, the superconductor UBe13 is of scientific interest because of the desire to understand the fundamental behavior of f-electrons in materials. UBe13 has already been studied by neutron powder diffraction and shown to scatter strongly with intense, sharp peaks. It crystallizes in the cubic NaZn13 -type structure, space group Fm3c. Samples made by different techniques, however, show different lattice parameters. UBe13 crystals can be made by arc-melting or single crystals can be grown in an aluminum flux. In previous literature, the Al-flux grown UBe13 has a consistently larger lattice parameter, well outside of error bars. We have recorded neutron powder diffraction data from two UBe13 samples in order to determine the crystallographic changes, if any, between arc-melted and flux-grown samples. Although analysis at the time of this writing is preliminary, we will present new data and discuss interpretations. If there are differences, it may be that small amounts of aluminum impurities from the processing environment have been incorporated into the samples. Another possible explanation is that the actual crystal structures are different between the two samples, since arc-melting produces this compound at much higher temperatures (approx. 2300C) than flux growth (approx. 1100C). But the raw data indicate this to be unlikely. Any impurities or crystallographic differences due to preparation would have a huge impact on the intrinsic electronic properties of the sample. In turn, these details must be accurately incorporated into theoretical models of electronic behavior. Neutron scattering is an ideal tool for investigating isotopically pure systems that would be difficult to study with X-ray techniques, such as those elements exhibiting extremes of X-ray scattering behavior like depleted uranium and beryllium. Whole-pattern Rietveld refinements and simulations will be used to analyze the data. This work was performed on the high-pressure preferred orientation diffractometer (HIPPO) at the Manuel Lujan Jr. Neutron Scattering Center, LANSCE, Los Alamos National Laboratory, a DOE User facility.
5:15 PM - **Z10.3
Onset of Antiferromagnetism in URu2Si2 Under Nearly Hydrostatic Conditions.
Nicholas Butch 1 , Jason Jeffries 2 , Jeffrey Lynn 3 , M. Maple 4
1 Center for Nanophysics and Advanced Materials, University of Maryland, College Park, Maryland, United States, 2 , Lawrence Livermore National Laboratory, Livermore, California, United States, 3 , NIST Center for Neutron Research, Gaithersburg, Maryland, United States, 4 Dept. of Physics, University of California, San Diego, La Jolla, California, United States
Show AbstractThe temperature-pressure phase diagram of the heavy fermion superconductor URu2Si2 has received a great deal of interest in recent years, motivated by questions about the relationship between the elusive hidden order phase at ambient pressure, and the pressure-induced long range antiferromagnetic order. Current understanding of the phase boundaries puts the zero-temperature critical pressure near 5 kbar, connected by a line of first-order transitions terminating somewhere between 10 and 15 kbar at the paramagnetic phase boundary at about 18 K. We performed a neutron scattering study under nearly hydrostatic conditions using helium as a pressure-transmitting medium up to pressures of 10 kbar. Under these conditions, the antiferromagnetic zero-temperature critical pressure is greater than 7 kbar and the magnetic transition temperature is still less than 15 K at 10 kbar.
Z11: Poster Session: Actinides
Session Chairs
Friday AM, April 09, 2010
Salon Level (Marriott)
9:00 PM - Z11.1
A Magnetic Investigation of Plutonium Americium Alloys and the Effects of Radiation Damage.
Scott McCall 1 , Michael Fluss 1 , Brandon Chung 1 , Richard Haire 2
1 , LLNL, Livermore, California, United States, 2 , ORNL, Oak Ridge, Tennessee, United States
Show AbstractPlutonium stands near the critical point between the itinerant electron behavior of the light actinides and the localized electrons of the heavy actinides for americium and beyond. This leads to a profusion of unusual properties that continue to defy theoretical explanation; thus far a number of theories resolve many of these properties correctly, but also predict magnetism in plutonium. Recent measurements have shown that alpha and delta Pu have a substantially equivalent 5f electron count of ~5.2, yet display neither magnetic ordering, nor the presence of local moments. Instead, Pu possesses a large Pauli susceptibility consistent with a narrow conduction band at the Fermi surface. However, it may be possible to induce magnetic behavior by applying an appropriate stress. One way to do so is to exploit the radioactive decay of Pu to create damage cascades of vacancies and interstitials within the lattice—a reversible process where damage may be removed by thermal annealing. Damage accumulated at low temperatures induces significant changes in the magnetic susceptibility indicative of local moments of ~30 Bohr magnetons per alpha decay in the dilute limit.A second method to perturb the system is to expand the lattice by alloying Pu with Am, which for concentrations of Am from 5-80% forms the fcc delta phase of Pu (and beta phase of Am). The 5f electrons of Am are fully localized, and from a magnetic perspective, Am is a nearly ideal alloying agent because it has an electronic structure corresponding to a filled sublevel (J=0) so there is no local moment present. As the lattice expands due to Am, the degree of overlap between neighboring Pu f-orbitals decreases and the corresponding bands become increasingly narrow. Additionally, the 5f electrons of Am are localized in a closed sublevel effectively removing electrons from the 5f-band. In moving towards localization the Pauli contribution should diverge, ultimately leading to the appearance of local moment magnetism as the 5f electrons become localized and the Pauli contribution drops dramatically. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.
9:00 PM - Z11.2
Actinide Epitaxial Thin Films Grown by Polymer-assisted Deposition.
Eve Bauer 1 , Thomas McCleskey 1 , Anthony Burrell 1 , Brian Scott 1 , Quanxi Jia 2
1 Materials Chemistry, Materials Physics and Applications, Los Alamos National Laboratory, Los Alamos, New Mexico, United States, 2 Superconductivity Technology Center, Materials Physics and Applications, Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Show AbstractMany thin film deposition techniques such as chemical vapor deposition (CVD) involve volatile precursors, which poses a safety concern when depositing actinide thin films. We will report here on the use of a water based solution technique known as polymer assisted deposition (PAD) for the formation of epitaxial actinide thin films. We have used this process to make stable, epitaxial uranium films in multiple oxidation states. UO2 films are obtained on a lanthanum aluminum oxide substrate and expitaxial U3O8 films on a single crystal c-plane Al2O3 substrate. The oxidation states of uranium can be “pinned down” simply by matching the substrate lattice to the film crystal lattice. The oxidation state of actinides plays a key role in the chemistry that can occur during the storage of radioactive materials. Studies on the surface chemistry of high quality thin films can provide fundamental information for modeling the complex issues in storage. For instance, the oxidation of UO2 generates a powder-like U3O8, which can cause the splitting of the storage sheath, impacting the safety of long term storage of actinides. PAD can be used to make other films such as nitrides by simply changing the atmosphere under which the films are annealed. The solution chemistry during the annealing process can dramatically change the final product. We will discuss how changing the solution chemistry can be used to yield uranium dinitride films under a mixed N2/H2 atmosphere and present conductivity and IR characterization of the films.
9:00 PM - Z11.3
Gas-phase Reactions of Uranate Ions With Methanol: A Convergence of Experiment and Theory.
Maria del Carmen Michelini 1 3 , Joaquim Marcalo 2 , Nino Russo 1 , John Gibson 3
1 Dipartimento di Chimica, Università della Calabria, Arcavacata di Rende Italy, 3 Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 2 Unidade de Ciências Químicas e Radiofarmacêuticas, Instituto Tecnológico e Nuclear, Sacavém Portugal
Show AbstractThe use of Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) to examine gas-phase reactions of actinide ions with elementary neutral molecular substrates has developed into an important tool to explore fundamental aspects of actinide chemistry [1,2]. There has been a substantial effort to understand the experimentally observed gas-phase reactions through the use of density functional theory (DFT), specifically B3LYP/SDD and PW91/TZVP using the ZORA approximation, to describe in detail the reaction pathways through which the observed reactions proceed [3-6]. In the work reported here, the theory and experimental efforts have been melded together to allow for the more effective application of both. A method to produce gas-phase uranate anions—UO3-, UO4-, etc.—by laser desorption ionization has been developed in our laboratory [7]. We are using FTICR-MS and DFT to explore bimolecular reactions of methanol with these elementary uranate anions [8]. The uranate/methanol reaction system was selected as it presents an opportunity to explore interactions between actinide oxides and organic molecules at a very basic level, absent perturbations found in the condensed phase. The DFT results describe the UOx-/methanol reaction mechanisms in detail. The kinetics and product distributions are determined by FTICR-MS studies of the primary and sequential reactions of the UOx- ions with methanol.A key aspect of this work is the use of isotopically labeled methanols—CD3OH, CH3OD and CH318OH—to experimentally probe reactant and product structures and follow reaction pathways, for direct comparison with the DFT results. These reactions, which were experimentally studied in an analogous manner to ascertain whether the reaction pathways provided by DFT are valid, will be described. There is good agreement between the experimental and theory results, on the whole validating the latter.*This work was supported by the Universitá degli Studi della Calabria; by Fundação para a Ciência e a Tecnologia (FCT) under contract PPCDT/QUI/58222/2004; and by the Director, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences of the U.S. Department of Energy under Contract DE-AC02-05CH11231 at LBNL.[1] Gibson J K and Marçalo J 2006 Coord. Chem. Rev. 250 776[2] Gibson J K, Haire R G, Marçalo J, Santos M, Leal J P, Pires de Matos A, Tyagi R, Mrozik M K, Pitzer R M and Bursten B E 2007 Eur. Phys. J. D 45 133[3] Michelini M C, Russo, N and Sicilia E 2006 Angew. Chem. Int. Ed. 45 1095 [4] Michelini M C, Russo, N and Sicilia E 2007 J. Am. Chem. Soc.129 4229[5] Mazzone G, Michelini M C, Russo N, and Sicilia E 2008 Inorg. Chem. 47 2083[6] Alikhani M E, Michelini M C, Russo N and Silvi B 2008 J. Phys. Chem. A 112 12966[7] Marçalo J, Santos M, Pires de Matos A and Gibson J K 2009 Inorg. Chem. 48 5055[8] Michelini M C, Marçalo J, Russo N and Gibson J K 2009 (unpublished results)
9:00 PM - Z11.4
Application of Secondary Ion Mass Spectrometry to the Characterization of Water Attack of UO2 Matrices.
Fabio Belloni 1 , Ilaria Marchetti 1 , Alice Seibert 1 , Thomas Gouder 1 , Rachel Eloirdi 1 , Clive Walker 1 , Emilio Maugeri 1
1 Institute for Transuranium Elements, European Commission - Joint Research Centre, Eggenstein-Leopoldshafen Germany
Show AbstractSecondary ion mass spectrometry is an ideal tool for the measurement of elemental composition and isotopic abundance in surface analysis, depth profiling and microstructural characterization of solids, thanks to its high sensitivity and spatial resolution [1]. In conjunction with SEM, high-resolution profilometry, and the use of a tracer (18O), we have exploited these capabilities to characterize UO2 matrices in different physical forms (e.g., polycrystalline pellet, thin film grown by sputter deposition, single crystal) with respect to the attack of water, under conditions relevant for final disposal of spent nuclear fuel in a geologic repository. In detail, ion imaging of 238U16O2- and 18O-, driven by a Cs+ primary beam, has been used for surface analysis, before and after static leaching experiments with 18O-labeled water. Depth profiles of the isotopic ratio 18O/(16O + 18O) provided basic data for the evaluation of oxygen and water diffusion coefficients in lattice and along grain boundaries, respectively [2]. The developed methodology can be extended to other polycrystalline nuclear waste materials [3] as well. The goal of our study on UO2 is twofold: on the one hand, to gain understanding on the mechanisms of fuel oxidation and water penetration; on the other hand, to assess how well thin films can reproduce physico-chemical properties of bulk samples. An overview of the most significant experimental results is provided, with a special focus on measurement issues and data analysis. In particular, a thorough measurement of the sputtering yield has been carried out in order to calibrate the length scale of the depth profiles (up to 20 µm). Measurements on different matrices and at different stoichiometries (UO2+x) have also been performed with a view to assessing whether the sputtering yield can be regarded as a figure of merit for the recognition of different crystalline phases and of oxidation state. With regard to depth profiling, the probing method is described, with which it is possible to discriminate between short-range, near-surface and long-range diffusion regime. Simple modelling of the raster-averaged profiles is proposed with a view to extract information about the 18O local concentration along grain boundaries, as well as about the wet surface.References[1] W. Gust et al., J. Physique, 46 (1985) C4-475[2] A.C.S. Sabioni et al., J. Nucl. Mat. 278 (2000) 364[3] R.C. Ewing et al., J. Mater. Res. 13 (1998) 1434
9:00 PM - Z11.6
Photoelectron and Fano Spectroscopy of U Oxide at LLNL.
James Tobin 1 , Sung Woo Yu 1 , Brandon Chung 1 , G Dan Waddill 2
1 Physical and Life Sciences, Lawrence Livermore National Laboratory, Livermore, California, United States, 2 Department of Physics, Missouri University of Science and Technology, Rolla, Missouri, United States
Show AbstractIn our laboratory, an effort is underway to investigate the underlying complexity of 5f electronic structure with spin-resolved photoelectron spectroscopy using chiral photonic excitation, i.e. Fano Spectroscopy. Our previous Fano measurements with Ce indicate the efficacy of this approach [1,2] and theoretical calculations and spectral simulations suggest that Fano Spectroscopy may resolve the controversy concerning Pu electronic structure and electron correlation. [3,4] To this end, we have constructed and commissioned a new Fano Spectrometer, [5] testing it with the relativistic 5d system Pt. [6,7] We will present our preliminary photoelectron spectra of the UO2 system and discuss our plans for the future.AcknowledgmentsLawrence Livermore National Laboratory is operated by Lawrence Livermore National Security, LLC, for the U.S. Department of Energy, National Nuclear Security Administration under Contract DE-AC52-07NA27344. This work was supported by the DOE Office of Basic Energy Science and Campaign 2/WCI/LLNL.References1.J.G. Tobin, S.W. Yu, T. Komesu, B.W. Chung, S.A. Morton, and G.D. Waddill, “Evidence of Dynamical Spin Shielding in Ce from Spin-resolved Photoelectron Spectroscopy,” EuroPhysics Letters 77, 17004 (2007).2.S.W. Yu, T. Komesu, B.W. Chung, G.D. Waddill, S.A. Morton, and J.G. Tobin, “f-electron correlations in nonmagnetic Ce studied by means of spin-resolved resonant photoemission,” Phys. Rev. B 73, 075116 (2006).3.S.W. Yu, J.G. Tobin, and P. Söderlind, “An Alternative Model for Electron Correlation in Pu,” J. Phys. Cond. Matter 20, 422202 (2008), Fast Track Communication. 4.J.G. Tobin, S.W. Yu, B.W. Chung and G.D. Waddill, “Resolving the Pu Electronic Structure Enigma: Past Lessons and Future Directions,” J. Nucl. Matl. 385, 31 (2009).5.J.G. Tobin, S.W. Yu, T. Komesu, B.W. Chung, S.A. Morton, and G.D. Waddill, “Facilities for the performance of Fano measurements as a probe of electron correlation,” Matl. Res. Soc. Symp. Proc. 986, 63 (2007).6.S.W. Yu and J.G. Tobin, “Observation of the underlying relativistic effect in the valence bands of Pt,” Surface Science Letters 601, L127 (2007).7.S.W. Yu and J. G. Tobin, “Breakdown of spatial inversion symmetry in core level photoemission, Phys. Rev. B 77, 193409 (2008).
9:00 PM - Z11.8
Structural Investigations on the Pu-Am-O System.
Elisabeth Gavilan 1 , Renaud Belin 1
1 DEN - DEC - SPUA - LMPC, CEA Cadarache, Saint Paul Lez Durance, 14999, France
Show AbstractDuring the fission process in nuclear reactors, Minor Actinides (MA) (Np, Am, Cm, …) are generated. MA being long-lived nuclides, their management is an important environmental issue. One possible management route for these elements, after they are partitioned from the bulk, is to “burn” them in a dedicated nuclear reactor. This process, called “transmutation”, converts them via neutron interactions into nuclei with shorter half-lives (if performed with fast neutrons), better suited for waste disposal. Among MA, Am is of main concern as its production increases with the fuel burn-up and cooling time [1]. Two strategies are currently considered to transmute MA in fast neutron systems (Fast reactors or Accelerator-Driven Systems): (a) incorporating small amounts of MA (e.g. <5%) in (U,Pu)O2 fuels or (b) mixing MA in higher amounts with pure uranium (blankets in fast reactor) or plutonium (transmutation fuels in ADS). For the latter, Pu-Am-O compounds are of great interest. The fundamental physical chemistry of the Pu-Am-O system is poorly known. Only Am-O [3] and Pu-O [4] binaries phase diagrams have been previously investigated. First, several AmxPu1-xO2 solid solutions were synthesized with x between 0.1 and 0.9 by solid state reaction under air at 1773K starting from pure 241AmO2 and PuO2 powders. Then, the resulting compounds were reduced under Ar/H2 (5%) at the same temperature. Due to the γ -irradiation of 241Am, syntheses were realized on a few milligrams scale. XRD analysis shows that Pu- and Am-rich compositions are respectively related to Pu2O3 (cubic, Ia-3 s. g.) and Am2O3 (hexagonal P-3m1 s. g.) structures. However, XRD patterns of intermediate compositions do not correspond to any known structure in the Pu-O and Am-O systems. Furthermore, a closer look at the different diffraction patterns reveals a new phase regardless the composition.In the presentation, a detailed structural analysis of the multiphasic system observed for each composition will be discussed. Our results provide relevant data that will allow to partially build the Pu2O3-Am2O3 phase diagram. Furthermore, these data should be useful to better define the choice of the composition of the future fuels for both 4th generation and ADS reactors.[1] Actinide and fission product partitioning and transmutation – Status and assessment report 2000 Nuclear Energy Agency OECD.[2] Accelerator Driven Systems (ADS) and Fast Reactors (FR) in Advanced Nuclear Fuel Cycles: A Comparative Study 2002 Nuclear Energy Agency OECD.[3] Chikalla T. D., Eyring L. 1968 J. Inorg. Nucl. Chem. 30 133.[4] Wriedt H. A. 1990 Bull. Alloy Phase Diagrams 11 184.
9:00 PM - Z11.9
Plutonium Environment in Lanthanide Borosilicate Glass.
Sergey Stefanovsky 1 , Andrey Shiryaev 2 , Yan Zubavichus 2 , James Marra 3
1 , SIA Radon, Moscow Russian Federation, 2 , RRC Kurchatov Institute, Moscow Russian Federation, 3 , Savannah River National Laboratory, Aiken, South Carolina, United States
Show AbstractThe sample of LaBS glass with a target chemical composition (wt %): 9.0 Al2O3, 11.8 B2O3, 12.2 Gd2O3, 6.3 HfO2, 17.2 La2O3, 13.6 Nd2O3, 9.5 PuO2, 18.1 SiO2, 2.3 SrO was prepared from PuO2 powder and mechanically activated mixture of chemicals at 1500 C. The obtained product was visually homogeneous. X-ray diffraction of the as-prepared glass showed that it mostly consists of a vitreous phase with minor crystalline PuO2 (or PuO2-HfO2 solid solution with minor HfO2) and britholite-type phase. X-ray absorption spectra of Pu LIII edge in the as-prepared and stored for various periods LaBS glasses were recorded, analyzed and compared with the spectra of crystalline PuO2, Pu in the as-prepared glass exists in predominantly tetravalent form (Pu4+ ions) but its storage in air results in partial oxidation as it is seen from shift of peak energy values. In the structure of the as-prepared glass Pu4+ ions have a co-ordination number (CN) close to 6 (~6.3) and are located within the axially squeezed octahedra with five equidistant oxygen ions at a distance of 2.265±0.015 A and one – at shorter distance (2.130±0.010 A) from the Pu4+ ion. The Pu-Pu(M) distance (second co-ordination shell) is 3.675±0.015 A. “Aging” of the LaBS glass with transformation of some fraction of Pu into penta- or/and hexavalent form is accompanied by a structure transformation. Average CN of Pu in the first shell reduces from ~6.3 to ~5.6-5.7. Pu having an environment with a configuration of axially squeezed tetragonal pyramid may be attributed to Pu(V) or Pu(VI) with CN=5 whereas major Pu (IV) has distorted octahedral environment with CN=6. Pu-Pu distance (~3.74 Å) in the glass structure remains same after 1, 1.5 and 2 yrs of storage.
Symposium Organizers
Scott McCall Lawrence Livermore National Laboratory
Eric Bauer Los Alamos National Laboratory
John Gibson Lawrence Berkeley National Laboratory
Thomas Fanghaenel European Commission Joint Research Center
Lynda Soderholm Argonne National Laboratory
Z12: Properties of Actinide Superconductors
Session Chairs
Friday AM, April 09, 2010
Room 3008 (Moscone West)
9:00 AM - **Z12.1
Anisotropy of Antiferromagnetic Spin Fluctuations in the Heavy Fermion Superconductors of CeMIn5 and PuMGa5 (M=Co, Rh, Ir).
Hironori Sakai 1 2 , Seung-Ho Baek 2 , Stuart Brown 3 , Shinsaku Kambe 1 , Yo Tokunaga 1 , Yoshinori Haga 1 , Eric Bauer 2 , Joe Thompson 2
1 Advanced Science Research Center, Japan Atomic Energy Agency, Tokai, Naka, Ibaraki, Japan, 2 , Los Alamos National Laboratory, Los Alamos, New Mexico, United States, 3 Department of Physics, University of California, Los Angeles, California, United States
Show AbstractThe anisotropy of antiferromagnetic spin fluctuations has been investigated microscopically in the heavy fermion systems of CeMIn5 and PuMGa5 (M=Co, Rh, Ir) by means of nuclear magnetic resonance (NMR). Both systems are known to be relatively high-Tc superconductors among the heavy fermion systems, especially PuCoGa5, which has a Tc=18.5 K almost one order of magnitude larger than for CeCoIn5 ( Tc=2.3 K). We found strong XY-type anisotropy in the antiferromagnetic spin fluctuations in the normal states of these unconventional superconductors. Moreover, as compared with the several related paramagnetic systems, including UCoGa5, UFeGa5, PuIrGa5 and antiferromagnets CeRhIn5, NpCoGa5, the materials with larger XY-type fluctuations have a higher Tc. We will discuss in more detail these new results based on NMR measurements.
9:30 AM - Z12.2
Hybridization and Superconducting Gaps in Heavy-fermion Superconductor PuCoGa5 Probed via the Dynamics of Photoinduced Quasiparticles.
Diyar Talbayev 1 2 , Kenneth Burch 3 , Elbert Chia 4 , Stuart Trugman 2 , Jian-xin Zhu 2 , Eric Bauer 2 , John Kennison 2 , Jeremy Mitchell 2 , Joe Thompson 2 , John Sarrao 2 , Antoinette Taylor 2
1 Chemistry, Yale University, New Haven, Connecticut, United States, 2 , Los Alamos National Laboratory, Los Alamos, New Mexico, United States, 3 Physics, University of Toronto, Toronto, Ontario, Canada, 4 Physics and Applied Physics, Nanyang Technological University, Singapore Singapore
Show AbstractWe have examined the relaxation of photoinduced quasiparticles in the heavy-fermion superconductor PuCoGa5 using femtosecond pump-probe spectroscopy. This study was conducted on a clean as-grown surface of PuCoGa5 single crystal grown in Ga flux. A 45-femtosecond optical pulse was used as an excitation (pump) that created a non-equilibrium electron distribution in the material. Electron-electron collisions thermalize the electronic distribution, after which electron-phonon collisions equilibrate electron and lattice temperatures. The material’s response is monitored using a weaker optical pulse (probe) that allows the measurement of pump-induced changes in the material’s reflectance (photoinduced reflectance). By varying the time delay between the pump and probe pulses, we can record the evolution of the photoinduced reflectance with time resolution ≤ 100 fs. The decay time of photoinduced reflectance in the normal state of PuCoGa5 corresponds to electron-lattice relaxation time τel, which is determined by the electron-phonon coupling constant λ. At room temperature, we measure τel=0.64 ps, which in PuCoGa5 corresponds to λ=0.20-0.26. Using the Allen-Dynes formula for the superconducting transition temperature with the effective Coulomb repulsion μ=0.1, we calculate Tc<0.03 K. This temperature is much lower than the experimental value of 18.5 K, which makes the measured electron-phonon coupling constant λ=0.26 incompatible with the measured Tc. Our result speaks against phonon-mediated superconducting pairing.Upon lowering the temperature in the normal state (T>Tc), we observe an order-of-magnitude increase of the photoinduced reflectance relaxation time, in agreement with a phonon bottleneck – evidence for the presence of a hybridization gap in the electronic density of states. The gap results from the hybridization between the conduction band and the localized Pu f-electron states. The relaxation of quasiparticles across the hybridization gap creates high-frequency phonons that can subsequently excite quasiparticles back into the states above the gap. Therefore, the decay of the excited quasiparticle population is governed by the decay of the high-frequency phonons, consistent with similar studies of other heavy-fermion metals. A kinetic model that describes the coupled quasiparticle and phonon populations allows the estimate of the hybridization gap magnitude to be ~90 K. Our measurement provides the first evidence for the presence of hybridization gap in PuCoGa5.The photoinduced reflectance in the superconducting state exhibits dramatic changes that we ascribe to the opening of the superconducting (SC) gap at the Fermi level. The observed dynamics confirms that the same quasiparticles detected in the normal state, i.e., the heavy quasiparticles, also participate in the SC pairing.
9:45 AM - Z12.3
Electronic Structure Theory of PuCoGa5 Bulk and Thin Films.
Alexander Shick 1 , Ladislav Havela 2
1 , Institute of Physics ASCR, Prague Czech Republic, 2 , Charles University, Prague Czech Republic
Show AbstractThe mechanism of superconductivity in Pu-based superconductor, PuCoGa5 (Tc= 18.5 K)remains to be uncovered. Detailed studies of physical properties of PuCoGa5 are essential in order to clarify the nature of the superconductivity and the role played by the 5f states. Photoemission (PES) spectroscopy is a powerful tool to study the nature of the 5f states. Up to date, the PES spectra for PuCoGa5 are still a matter of dispute. The difference between two observations [1,2] is likely due to a difference in the compositon of the top layer, affected by a Ga segregation [1]. The most apparent marker of approaching the correct stoichiometry is the gradual loss of the Fermi level emission. In this work, we study theoretically the electronic structure and PE spectra of PuCoGa5 making use of the LDA+Hubbard I approximation (LDA+HIA) implemented in the full-potential LAPW basis, including self-consistency over the charge density [3]. The calculated PE spectra of PuCoGa5 show relative reduction of the spectral weight at the Fermi edge and slight shift of f-peak positions in comparison with δ-Pu. These changes are correlated with an increase of f-manifold occupation nf (nf=5.3 in PuCoGa5 vs. nf=5.2 in δ-Pu). There is fairly good agreement between calculated PE spectra and experimental results of [1] for the bulk PuCoGa5. Furthermore, we study the electronic structure of thin PuCoGa5+x films in order to determine the effect of Ga exess at the surface. The results will be compared with calculations performed for the δ-Pu metal using the same method. [1] Eloirdi R, Havela L, Gouder T, et al., J. Nucl. Mater. 385, 8 (2009). [2] J.J. Joyce, J.M. Wills, T. Durakiewicz, M.T. Butterfield, E. Guziewicz, J.L. Sarrao, L.A. Morales, A.J. Arko, O. Eriksson., Phys. Rev. Lett. 91, 176401 (2003). [3] A. B. Shick, J. Kolorenc, L. Havela, and A. I. Lichtenstein, Phys. Rev. B 80, 085106 (2009).
10:00 AM - Z12.4
Evolution of the Magnetic and Superconducting States in UCoGe With Fe and Ni Substitution.
James Hamlin 1 , Noravee Kanchanavatee 1 , Kevin Huang 1 , Ryan Baumbach 1 , Diego Zocco 1 , Lei Shu 1 , Marc Janoscheck 1 , M. Maple 1
1 Department of Physics, University of California, San Diego, La Jolla, California, United States
Show AbstractThe very small number of known ferromagnetic superconductors places the study of such compounds at the frontier of superconductivity research. Recently, UCoGe has emerged as a new member of the class of materials exhibiting coexistence of ferromagnetism and superconductivity (Curie temperature TCurie = 3 K; superconducting critical temperature Tc = 0.8 K). This compound has generated much excitement in part because it has been proposed that the superconductivity derives from spin triplet pairing mediated by critical fluctuations of the magnetic order parameter. Therefore, a key question is how changes in the magnetic state of UCoGe affect the superconducting properties. Like the other known U-based compounds that exhibit coexistence of ferromagnetism and superconductivity (UGe2, URhGe, and UIr), the ferromagnetism in UCoGe is apparently itinerant in nature. We have carried out a comprehensive study of the UCo1-xFexGe and UCo1-xNixGe series of compounds across the entire range of composition 0 ≤ x ≤ 1. In this talk, we report the results of x-ray diffraction, electrical resistivity, magnetization, and specific heat measurements to elucidate the magnetic and superconducting phase diagram of the U[Fe, Co, Ni]Ge system. Substitution of either Ni or Fe into UCoGe initially results in an increase in the Curie temperature and the ratio of the effective moment to the saturation moment µeff/µsat. This ratio is widely regarded as a measure of itinerant ferromagnetism, with larger values indicating a greater degree of itineracy. At higher substituent concentrations (x), the ferromagnetic state crosses over to paramagnetism in UCo1-xFexGe and antiferromagnetism in UCo1-xNixGe. The interplay between the changing magnetic properties and superconductivity is discussed.
10:15 AM - Z12.5
Investigation of Structure/Property Relationships in UCoGe and Other Correlated Electron Systems.
Paul Tobash 1 , Krzysztok Gofryk 1 , Filip Ronning 1 , Joe Thompson 1 , Roman Movshovich 1 , Corneliu Miclea 1 , Eric Bauer 1
1 MPA-10, Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Show Abstract We present further results on the physical properties on the new ferromagnetic superconductor UCoGe including magnetization, specific heat, and electrical resistivity measurements. The investigation allows for a better understanding for studying the superconductivity order parameter symmetry along with how the superconductivity is connected with the observed magnetism. These results were obtained from high quality single-crystal samples, having RRR values of 10-35, grown using the Czochralski technique and subsequent electro-refinement/zone refinement efforts. UCoGe crystallizes with the TiNiSi structure type in the orthorhombic space group Pnma. Its structure can be envisioned as being composed of puckered three-bonded CoGe sheets which are interspaced with U atoms. The layered structure is similar to that found for the ferromagnetic superconductor UGe2, which is built from square sheets and zig-zag chains of Ge atoms interspaced with U atoms. Both of these compounds, along with URhGe, i.e. isostructural with UCoGe, belong to a growing class of ferromagnetic superconductors, in which the long-range ferromagnetic order may lead to spin-triplet superconductivity. UCoGe was found to order ferromagnetically at 3 K and further exhibits superconductivity below 0.7 K at ambient pressure. The ordered moment is very small, on the order of 0.07 μB per formula unit, suggesting itinerant ferromagnetism.
10:30 AM - **Z12.6
Spin Fluctuations in Superconducting CeCoIn5 and Antiferromagnetic CeRhIn5.
Chris Stock 1
1 , ISIS Facility, Chilton, Didcot United Kingdom
Show AbstractThe CeXIn5 (with X=Rh,Ir and Co) are metallic heavy fermion systems which display a delicate balance between antiferromagnetism and superconductivity [1]. While CeCoIn5 has the highest superconducting transition (Tc=2.3 K) of any known heavy fermion compound, CeRhIn5 is an antiferromagnet (TN=3.8 K) with bulk superconductivity only below 0.1 K [2]. The presence of unusually high temperature superconductivity and two-dimensional planes of magnetic Ce3+ ions links these systems to other unconventional superconductors such as the cuprates [3]. The quasi two-dimensional nature of these compounds is reflected in the electronic properties measured by quantum oscillation experiments. I will present a series of experiments investigating the magnetic fluctuations in CeCoIn5 and CeRhIn5 measured with the use of neutron inelastic scattering. I will show that the magnetic fluctuations in the superconducting state of CeCoIn5 are dominated by a “resonance” peak located in momentum at Q=(1/2,1/2,1/2) [4]. I will present studies investigating the properties of the spin fluctuations as a function of both temperature and magnetic field. Comparisons to antiferromagnetic CeRhIn5 will be made. These results demonstrate the direct correlation between magnetic fluctuations and superconductivity. [1] C. Petrovic et al., J. Phys. Condens. Matter 13, L337 (2001).[2] J. Paglione et al., Phys. Rev. B 77, 100505(R) (2008).[3] R.J. Birgeneau et al., J. Phys. Soc. Jpn., 75, 111003 (2006).[4] C. Stock et al., Phys. Rev. Lett. 100, 087001 (2008).
11:00 AM - Z12: An SCs
BREAK
Z13: Actindes and Magnetism
Session Chairs
Friday PM, April 09, 2010
Room 3008 (Moscone West)
11:30 AM - Z13.1
Breakdown of the Antiferromagnetic Quantum Critical Point Scenario in the Noncentrosymmetric Heavy Fermion Compound Yb2Fe12P7.
Ryan Baumbach 1 , James Hamlin 1 , Lei Shu 1 , Diego Zocco 1 , M. Maple 1 , Pei Ho 2 , Jim O'Brien 3
1 Physics, University of California, San Diego, La Jolla, California, United States, 2 Physics, California State University, Fresno, Fresno, California, United States, 3 , Quantum Design, USA, San Diego, California, United States
Show AbstractPronounced deviations from Fermi liquid (FL) behavior are often observed in d- and f-electron compounds based on transition metal, lanthanide, and actinide elements with unstable valences. An interesting subset of these materials are correlated electron metals which exhibit so-called non-Fermi-liquid (NFL) behavior in the normal state physical properties: e.g., electrical resistivity ρ(T) ~ Tn (n < 2), specific heat divided by temperature C(T)/T ~ Tn (n < 1) or -lnT, magnetic susceptibility χ(T) = M/H ~ Tn (n < 1) or -lnT, and dynamical susceptibility χ''(T) = f(ω/T). Several routes to NFL behavior have been proposed including Kondo disorder, Griffith's phase, quadrupolar Kondo, and quantum critical point (QCP) models. The QCP model has been widely applied to situations where a second order phase transition, usually magnetic, is suppressed to T = 0 K by a control parameter such as chemical composition (x), pressure (P), or magnetic field (H), terminating in a QCP at T = 0 K. Order parameter fluctuations are manifested as NFL behavior at temperatures above the QCP and a line emanating from the QCP delineates a crossover from NFL to FL behavior at lower T and higher values of the control parameter. Extensive investigations of several prototypical systems (e.g. CeCu6-xAux and YbRh2Si2) which exhibit antiferromagnetic (AFM) order tuned by composition, pressure, and/or magnetic field have been analyzed within the context of the QCP model and support the predicted "V-shaped" temperature - control parameter phase diagram. We report new measurements on single crystals of a ternary compound with an AFM Yb sublattice, Yb2Fe12P7. The behavior that we observe in this compound does not conform to the widely accepted QCP scenario described above, in that the NFL behavior extends far beyond the QCP as the tuning parameter, H, is increased. In particular, we find: (1) an AFM ground state (TN ~ 0.9 K) which is suppressed towards a putative AFM QCP near H = 1.5 T, (2) a FL at temperatures below TN with a large zero field electronic mass, and (3) a crossover from the FL to an NFL regime at the AFM QCP, where the region of NFL behavior is increasingly stabilized (n approaches 1.5) for large values of H, in contrast to the widely accepted QCP scenario. These observations show that the simple QCP picture given above does not adequately describe the underlying physics of the Yb2Fe12P7, suggesting that the QCP scenario may need to be revised. We note that there are also several U-based systems (e.g., URu2-xRexSi2, U1-xThxPd2Al3), which also do not conform to the AFM QCP scenario for NFL behavior. Crystal synthesis was funded by the US Department of Energy under research grant DE FG02-04ER46105. The low-temperature measurements were funded by the National Science Foundation under grant 0802478.
11:45 AM - **Z13.2
Memory Effects and Slow Relaxation in Actinide-based Molecular Nanomagnets.
R. Caciuffo 1 , N. Magnani 1 , C. Apostolidis 1 , E. Colineau 1 , J. Griveau 1 , R. Eloirdi 1 , S. Cornet 2 , I. May 2 , C. Sharrad 2 , D. Collison 2 , R. P. Winpenny 2
1 , European Commission, Joint Research Centre, Institute for Transuranium Elements, Karlsruhe Germany, 2 School of Chemistry , The University of Manchester, Manchester United Kingdom
Show AbstractMolecular nano-magnets (MNM) are quasi-zero-dimensional objects offering unique opportunities to investigate magnetic properties at the transition between the quantum and classical regimes. Actinide-based single- and poly metallic complexes are very promising systems in this respect, for they are expected to exhibit higher anisotropy energy barriers than transition metals, and larger exchange coupling than rare-earth trivalent ions. Here we present the magnetic characterization of a heterovalent Np trimetallic cluster showing superexchange between the transuranic centers, and the first observation of low temperature magnetic memory effects in a transuranium molecular complex, neptunocene. The first compound can be schematized as an isosceles triangle, formed by one Np(VI) and two Np(V) centers. Magnetic susceptibility and magnetization curves can be modelled assuming a dominant axial ligand field term on the Np ions, antiferromagnetic superexchange coupling between Np ions with different valence, and a significantly weaker ferromagnetic exchange between the two Np(V). Studies of the dynamic magnetic behaviour show slow relaxation of the magnetization with an energy barrier of 140 K, which is nearly twice the size of the highest barrier found in polymetallic clusters of the d-block. Low-temperature magnetic memory effects combined with slow relaxation have been observed in neptunocene, a molecule composed of a single Np(IV) ion lying within two planar cyclooctatetraenyl rings in a "sandwich" structure with D8h symmetry. The degeneracy of the 5f3 configuration is partially removed by the axial ligand-field potential, leading to a well-isolated Jz = ± 5/2 doublet. The temperature dependence of the characteristic relaxation time can be fitted assuming a dominant Orbach process involving excited ligand-field states, perturbed by a direct relaxation channel between the two Zeeman-split components of the ground-state doublet.
12:15 PM - Z13.3
X-Ray Emission Spectroscopy Studies of f-Electron Systems at High Pressure: Actinides.
Kevin Moore 1 , William Evans 1 , Maganus Lipp 1 , P. Chow 2 , Y. Xiao 2 , R. Haire 3
1 Chemistry and Matrials Science, Lawrence Livermore Nationa Labs, Livermore, California, United States, 2 High-Pressure Collaboration Access Team (HPCAT), Geophysical Laboratory, Carnegie Institution of Washington, c/o Advance Photon Source, Argonne Natl Lab, Argonne, Illinois, United States, 3 , Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show AbstractThe physics of f-electrons drives a variety of unusual phase transitions and properties within the lanthanide and actinide series. Recent works by Moore et al. [1, 2] and others establish the importance of properly accounting for spin polarization effects in theoretical models; neglecting magnetic effects, theoretical studies find the energies of competing crystal structures to be nearly degenerate. The magnetic interactions have a strong effect on the stability of a particular phase and drive the associated richness of phases and phase transitions.X-ray emission spectroscopy (XES) has been used as a sensitive probe of pressure-induced changes in the local moments in f-electron system, notably Gadolinium [2].We will present our XES measurements of actinides, namely curium and proctactinium, at high pressure. It has been convincingly argued that the structural transitions in curium are accompanied by changes in the magnetic properties. Work of Moore et al. predicts a reduction in the magnetism with increasing pressure, though the moments persist up to modest pressure and in fact serve to stabilize the Cm-III structure. A strong degeneracy in energy is found among low pressure structures, with the electronic exchange interaction as the driving mechanism for the observed structures. An experimental test of these predictions using XES will test their interpretation and advance our understanding of the f-electron physics.References[1] K. T. Moore and G. van der Laan, Rev. Mod. Phys., 81, 235 (2009).[2] P. Söderlind and K. T. Moore, Scripta Materialia, 59, 1259 (2008).[3] K. T. Moore et al., Phys. Rev. Lett., 98, 236402 (2007).[4] S. Heathman et al., Nature, 309, 110 (2005).
12:30 PM - Z13.4
High Pressure Structural Phase Transformations in Curium, Americium, Berkelium and Californium Metals.
Rajeev Ahuja 1
1 Physics and Materials Science, Uppsala University, Uppsala Sweden
Show AbstractDensity-functional electronic structure calculations have been used to investigate the high pressure behavior of curium (Cm), americium (Am, berkelium (Bk) and Californium (Cf) metals. The phase transitions from fcc to the low symmetry structures are shown to originate from a drastic change in the nature of the electronic structure induced by the elevated pressure. For the low density phases, an orbital polarization correction to the local spin density(LSD) theory was applied. Gradient terms of the electron density were included in the calculation of the exchange/correlation energy and potential, according to the generalized gradient approximation (GGA). Theory compares rather well with recent experimental data which implies that electron correlation effects are reasonably modeled in our orbital polarization scheme.