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The effect of the addition of an organic monolayer to the surface of a clay
mineral on the speciation of metal ions intercalated into the clay
interlayer is probed by X-ray absorption spectroscopy. The presence of the
monolayer changes the surface of the clay from hydrophilic to hydrophobic.
It inhibits the interlayer ions from exchanging freely into environmental
water and reduces the leach rate of cations out of the clay by approximately
a factor of 20. Significant changes are observed when these coated samples
are treated under hydrothermal and thermal conditions. Reductions of
uranium(VI), in the form of uranyl, and copper(II) ions occur. In addition,
the uranium aggregates, forming small particles that appear similar to
UO2. Comparable conglomeration occurs with lead cations and
with the reduced copper species.
Uranium L3 X-ray absorption data were obtained from two
borosilicate glasses, which are considered as models for radioactive
wasteforms, both before and after leaching. Surface sensitivity to uranium
speciation was attained by a novel application of simultaneous fluorescence
and electron-yield detection. Changes in speciation are clearly discernible,
from U(VI) in the bulk to (UO2)2+-uranyl in the
corrosion layer. The uranium concentrations within the corrosion layer also
show variations with leaching times that can be determined from the
Single crystals of Np2O5 have been synthesized by low-temperature hydrothermal reaction of a Np5+ stock solution with natural calcite crystals. The structure of Np2O5 was solved by direct methods and refined on the basis of F2 for all unique data collected on a Bruker X-ray diffractometer equipped with an APEX II CCD detector. Np2O5 is monoclinic, space group P2/c, with a = 8.168(2) Å, b = 6.584(1) Å, c = 9.3130(2) Å, β = 116.01(1)˚, V = 449.8(2) Å3, and Z = 1. The structure contains chains of edge-sharing neptunyl pentagonal bipyramids linked into sheets through cation-cation interactions with distorted neptunyl square bipyramids. Additional cation-cation interactions connect the sheets into a three-dimensional framework. The formation of Np2O5 on the surface of calcite crystals has important implications for the precipitation of isolated neptunyl phases in natural aqueous systems.
We have investigated the structure and composition of La2CuO4 (both pure and Sr-doped) and Nd(Ba2−xNdx)Cu3O7 using powder neutron diffraction. In La2−xSrxCuO4, Tc reacnes its maximum at x=0.15, after which vacancies are introduced in the 2D planes and superconductivity is destroyed. Superconductivity in La2CuO4 is not bulk. No evidence for nonstoichlometry in this material can be observed. In the Nd(Ba2−xNdx)Cu3O7 system, Tc is a complex function of oxygen ordering and electronic effects.
We report the synthesis of Cm2CuO4. The lattice constants of this material, determined by x‐ray diffraction, show it to be a new member of the isostructural series R2CuO4 (R=Pr, Nd, Sm, Eu, and Gd). Analysis of magnetic measurements is consistent with a free‐ion effective moment for Cm3+, with no contribution to the susceptibility from Cu‐ions.
The question of valence of Cu and rare-earth atoms in the newly discovered oxide superconductors with high transition temperatures is crucial to the understanding of their electronic structure. We have measured the X-Ray Absorption Near Edge Structure (XANES) of Cu K transition and Pr Ljjj transition to obtain information on the valence of Cu, and Pr in La2−xSrxCuO4 and Y1−xPrxBa2Cu3O7-δ. We will present the experimental results, compare them with calculations of absorption cross-section for model atomic clusters, and discuss the valence of Cu as a function of oxygen concentration.
High-energy x-ray scattering (HEXS) is used to probe correlations about selected actinide ions in solution. The experiments were designed to favor homoleptic water coordination in order to compare the preferred structural environment of UO22+, NpO2+, and Cm3+ with similar ligands. In addition to comparisons between the three actinide ions in solution, the results from the HEXS studies are also compared with known solid state structures. Marked similarities are found between near- and next-near neighbor coordination in solution and in the solid state.
The hydrolysis chemistry of the tetravalent actinides is discussed based on recent studies with thorium. The coupling of high energy x-ray scattering and single crystal diffraction has provided insight into the structures of the polynuclear complexes formed by hydrolytic reactions of the tetravalent actinides. The success of these experiments with thorium presents many opportunities for the elucidation of the structures and thermodynamic quantities describing the dissolved polynuclear complexes of the heavier actinides.
High energy x-ray scattering (HEXS) is becoming a valuable tool for quantifying actinide coordination environments in solution. The technique is described with and exemplified by a study on Th speciation. A comparison of data from two samples with pHs of 0 and 3.4 show differences in the Th structural environment that can be attributed to Th colloid formation at the higher pH. A further comparison of the HEXS data from the colloidal sample with Th L3-edge EXAFS data on the same sample clearly demonstrates the advantage of the scattering data when looking for correlations at second-near neighbor or longer distances.
Recently uranyl peroxide nanoclusters containing 24, 28, and 32 uranyl polyhedra were chemically and structurally characterized under alkaline conditions. Such nanoclusters could conceivable form from oxidative alteration of nuclear waste in a geological repository by incorporating peroxide formed by alpha-radiolysis of water or in tanks where high-level waste is stored. The stability and persistence of uranyl peroxides in the vadose zone will be impacted by their interaction with geological media. Here we report batch experiments of solutions containing monodisperse nanoclusters in contact with crushed welded tuff. Within the first 72 hours, U concentrations in solution remained unchanged; however concentrations of Si, Al, Ca, Mg, Na, and Fe increased due to the dissolution of calcium-bearing aluminum silicate minerals in the welded tuff. Despite the presence of excess Li+ in solution, within two weeks crystals precipitated in which Ca2+ replaced Li+ in the nanocluster cage.
Np5+ sulfate compounds were synthesized using hydrothermal methods and their structures were determined using single crystal X-ray diffraction. Several compounds were found to contain cation-cation interactions between the Np polyhedra. Magnetic susceptibility measurements of two Np5+ sulfate compounds containing cation-cation interactions exhibit ferromagnetic ordering. The presence of the cation-cation interactions may provide a superexchange pathway through which magnetic interactions can be enhanced.
L3-x-ray absorption spectroscopy is used together with inelastic neutron scattering and magnetic susceptibility measurements to characterize the Tb oxidation state and bonding in the high-Tc related materials Y1-xTbxBa2Cu3O7 and Pb2Sr2TbCu3O8. The Tb is found to be essentially trivalent in both compounds with no indications of significant hybridization. However, there is evidence of significant Tb-Tb magnetic interactions in Pb2Sr2TbCu3O8 that persist to temperatures much higher than the Tb long-range ordering temperature.
Inelastic neutron scattering (INS) results on the intermultiplet transitions J=0 → 1 and J=1 → 2 in optically opaque EuBa2Cu3O7 are reported. Whereas these multiplets are split by the crystalline electric field (CEF), their low J values are influenced to first order only by the second-order (J=1) and additional fourth-order (J=2) CEF parameters. and the spinorbit coupling parameter were obtained by fitting the splitting of the J=l multiplet and the energy separation between the J=0 and 1 multiplets. The J=0 → 1 splitting observed here is smaller than previously seen by optical spectroscopic studies on a variety of transparent, ionic compounds, necessitating fitting of the free-ion parameter. Additional spectroscopic information on the J=2 multiplet indicates that additional fitting of free ion parameters must be included to adequately model the observed low energy separation between the two lowest J-multiplets. Preliminary calculation on the Q-dependence of the CEF split J=0 → 1 transitions and the comparison with observations are presented.
Polarized X-ray absorption studies have been carried out at the Cu K-edge to study the effect of Sr doping in La2CuO4 and oxygen doping in YBa2Cu3O6+x. These measurements help to elucidate the transitions giving rise to the absorption edges. We offer an explanation of the polarization shifts of features in terms of the results of our embedded cluster calculations of electronic structure.
Na+ in the Preyssler heteropolytungstate anion [NaP5W30O110]14− can be exchanged for a trivalent lanthanide ion. The potential significance of this new class of lanthanide heteropolyanions relates to their applications in catalysis science. This view follows from the fact that Keggin heteropolyanions and their free acids are used as heterogeneous solid catalysts and homogeneous solution catalysts. We describe synthetic conditions that lead to the incorporation of Ce3+ and Pr3+ within the Preyssler anion, and the coprecipitation of Ce3+ and the Preyssler anion. Initial studies indicate that the latter, coprecipitated, material deserves study for bifunctional catalytic activity.
x-ray absorption spectroscopy (XAS) has been used to investigate the structure and valence of thorium (Th4+) and uranyl () cations exchanged into two classes of microporous aluminosilicate minerals: zeolites and smectite clays. XAS is also employed to examine the fate of the exchanged cations after modification of the mineral surface using self-assembled organic films and/or exposure to hydrothermal conditions. These treatments serve as models for the forces that ultimately determine the chemical fate of the actinide cations in the environment. The speciation of the cations depends on the pore size of the aluminosilicate, which is fixed for the zeolites and variable for the smectites.
Biotic Np(V) reduction is studied in light of its potential role for the environmental immobilization of this hazardous radionuclide. The speciation of Np in Desulfovibrio desulfuricans cultures is compared with Np speciation in the spent medium and in the uninoculated medium. Precipitates formed in all three samples. Optical spectroscopy, x-ray diffraction, and x-ray absorption near edge structure (XANES) were used to determine the Np speciation. After 5 days of incubation, there was very little Np left in solution, which was present as Np(V). The precipitate that formed in all samples is an amorphous Np(IV) species, establishing that Np(V) is almost quantitatively reduced. These results demonstrate that the reduction of Np is independent of Desulfovibrio desulfuricans. The underlying chemistry associated with these results is discussed.
The ζ-phase, existing between 35 and 70% U in Pu, belongs to the high density phases seen from the point of view of systematics of allotropic modifications of Pu metal. Despite the volume per actinide atom only slightly higher than for α-Pu, it magnetic susceptibility is much higher than for α-Pu and exceeds even the δ-Pu value. Similarly, the Sommerfeld coefficient γ > 40 mJ/mol Pu K2 exceeds the experimental δ-Pu value. The data confirm that the volume is not the primary control parameter affecting the situation around the Fermi level of common Pu phases and they point against the traditional belief that they are essentially narrow 5f band systems.
Soft x-ray scanning transmission x-ray microscope (STXM) spectromicroscopy has been developed and employed to investigate several aspects of actinide chemistry and materials science at the Advanced Light Source Molecular Environmental Science (ALS-MES) Beamline 11.0.2 STXM end station. The basic approach and fundamentals of STXM experiments for radioactive materials systems is discussed. Representative results from STXM spectromicroscopy investigations of a mixed phase uranium nitride, single crystals of Eu(III)[TREN(Me-3,2-HOPO)]3 2H2O and hydrated Pu2(III)(C2O4)3(6H2O) 3H2O complexes are presented. The STXM images and soft x-ray absorption spectra illustrate the capabilities and utility of soft x-ray STXM for providing information about actinide materials, especially the light element constituents. Lastly, new and future opportunities for actinide science utilizing soft x-ray STXM are discussed in light of the planned upgrades for the STXM end stations at the ALS.
The evolution of pore and grain structure in a nuclear fuel environment is strongly influenced by the local temperature, and the temperature gradient. The evolution of pore and grain structure in an externally imposed temperature gradient is simulated for a hypothetical material using a Potts model approach that allows for porosity migration by mechanisms similar to surface, grain boundary and volume diffusion, as well as the interaction of migrating pores with stationary grain boundaries. First, the migration of a single pore in a single crystal in the presence of the temperature gradient is simulated. Next, the interaction of a pore moving in a temperature gradient with a grain boundary that is perpendicular to the pore migration direction is simulated in order to capture the force exerted by the pore on the grain boundary. The simulations reproduce the expected variation of pore velocity with pore size as well as the variation of the grain boundary force with pore size.