Crystalline Uranium Research Articles

Mechanisms of uranium interactions with hydroxyapatite: implications for groundwater remediation.

The speciation of U(VI) sorbed to synthetic hydroxyapatite was investigated using a combination of U LIII-edge XAS, synchrotron XRD, batch uptake measurements, and SEM-EDS. The mechanisms of U(VI) removal by apatite were determined in order to evaluate the feasibility of apatite-based in-situ permeable reactive barriers (PRBs). In batch U(VI) uptake experiments with synthetic hydroxyapatite (HA), near complete removal of dissolved uranium (>99.5%) to <0.05 microM was observed over a range of total U(VI) concentrations up to equimolar of the total P in the suspension. XRD and XAS analyses of U(VI)-reacted HA at sorbed concentrations < or = 4,700 ppm U(VI) suggested that uranium(VI) phosphate, hydroxide, and carbonate solids were not present at these concentrations. Fits to EXAFS spectra indicate the presence of Ca neighbors at 3.81 A. U-Ca separation, suggesting that U(VI) adsorbs to the HA surfaces as an inner-sphere complex. Uranium(VI) phosphate solid phases were not detected in HA with 4700 ppm sorbed U(VI) by backscatter SEM or EDS, in agreement with the surface complexation process. In contrast, U(VI) speciation in samples that exceeded 7000 ppm sorbed U(VI) included a crystalline uranium(VI) phosphate solid phase, identified as chernikovite by XRD. At these higher concentrations, a secondary, uranium(VI) phosphate solid was detected by SEM-EDS, consistent with chernikovite precipitation. Autunite formation occurred at total U:P molar ratios > or = 0.2. Our findings provide a basis for evaluating U(VI) sorption mechanisms by commercially available natural apatites for use in development of PRBs for groundwater U(VI) remediation.

Characterization of uranium oxide thin films grown from solution onto Fe surfaces

A novel method has been discovered for growing uranium oxide films onto iron substrates from solution. The films were characterized by X-ray photoelectron spectroscopy, X-ray diffraction, scanning electron microscopy, Rutherford backscattering spectrometry, and near-edge X-ray absorption fine structure. The as-grown films appear to be iridescent. They are composed of an amorphous uranium(VI) oxide with water incorporated, and are most likely a partially dehydrated schoepite. Topographic images reveal that the surfaces of the films are basically flat, but contain some small hills and valleys. Small cracks are distributed randomly across the surfaces. Upon heating in vacuum, the films crystallize and reduce to a uranium(IV) oxide, and the cracks enlarge. When a heated sample is exposed to air, the surface re-oxidizes to uranium(VI) while the bulk remains as crystalline uranium(IV) oxide.

All-electron LCGTO calculations for uranium dioxide

The linear combinations of Gaussian-type orbitals–fitting function (LCGTO-FF) technique is used to study the zero-pressure properties of crystalline uranium dioxide, a prototypical heavy element bearing Mott–Hubbard insulator. The effects of several common approximations are examined: (1) scalar vs. full relativity; (2) paramagnetic vs. spin polarized; and (3) local density approximation (LDA) vs. generalized-gradient approximation (GGA). UO2 is incorrectly predicted to be a metal for all approximations considered here, as expected for a Mott–Hubbard insulator. The lattice constant and bulk modulus obtained using the highest level approximations are, however, in excellent agreement with experiment. This suggests that the strong correlation effects that are generally believed to produce the observed band gap do not have a significant impact on the binding properties of UO2, contrary to the results of recent LDA+U calculations. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 80: 824–830, 2000

Structural properties of crystalline uranium from linear combination of Gaussian-type orbitals calculations*This article was performed under the auspices of the U.S. Department of Energy.

Scalar-relativistic linear combinations of Gaussian-type orbitals–fitting function (LCGTO–FF) calculations, using the local density approximation (LDA) and generalized gradient approximation (GGA) to density functional theory, were used to determine the atomic volumes, bulk moduli, and relative stabilities of uranium in the fcc, bcc, and α-U (orthorhombic) phases. The α-U phase is found to be the most stable, in agreement with experiment. The LDA yields an atomic volume that is about 7% smaller than experiment, while the GGA produces near perfect agreement with experiment. The numerical stability of the LCGTO–FF results is tested by comparison with a similar series of scalar–relativistic calculations using the full-potential linearized augmented plane-wave (FLAPW) method. The results obtained with the two methods are in excellent agreement, demonstrating the ability of the LCGTO–FF method to address the properties of a light actinide metal in its low-symmetry α phase. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 75: 911–915, 1999

Structural properties of crystalline uranium from linear combination of Gaussian-type orbitals calculations

Scalar-relativistic linear combinations of Gaussian-type orbitals-fitting function (LCGTO-FF) calculations, using the local density approximation (LDA) and generalized gradient approximation (GGA) to density functional theory, were used to determine the atomic volumes, bulk moduli, and relative stabilities of uranium in the fcc, bcc, and {alpha}-U (orthorhombic) phases. The {alpha}-U phase is found to be the most stable, in agreement with experiment. The LDA yields an atomic volume that is about 7% smaller than experiment, while the GGA produces near perfect agreement with experiment. The numerical stability of the LCGTO-FF results is tested by comparison with a similar series of scalar-relativistic calculations using the full-potential linearized augmented planewave (FLAPW) method. The results obtained with the two methods are in excellent agreement, demonstrating the ability of the LCGTO-FF method to address the properties of a light actinide metal in its low-symmetry {alpha} phase.

Harper-Dorn and power law creep in uranium dioxide

The creep behavior of uranium dioxide is reviewed based on the results of seven separate investigations. The data fall into two regimes: a power-law region at high stresses where the stress exponent, n, is about five and a Newtonian-viscous region at low stresses where the stress exponent is unity. It is shown that the data in the region where n = 1 cannot be explained by a Coble diffusional creep mechanism. This result negates the recent conclusion of Knorr, Cannon, and Coble who claim that “it is conclusively shown that Coble creep is the dominant mechanism” in the viscous region. It is demonstrated, rather, that Harper-Dorn creep describes the creep behavior of polycrystalline UO 2 above 0.6 T m and in single crystalline material in the region where n =1. Harper-Dorn creep in uranium dioxide is analyzed using an internal stress model developed to describe pure metal behavior. The value of the modulus-compensated internal stress, which is a measure of the random dislocation density, is shown to be higher for both polycrystalline and single crystalline uranium dioxide than it is for pure metals.

Surface behaviour of uranium dioxide after immersion in mineral water studied by X-ray photoelectron spectroscopy

X-Ray photoelectron spectroscopic (XPS) analysis has been carried out on samples of crystalline uranium dioxide following exposure to deionised or mineral water at 60 and 90°C for several weeks. A surface layer formed on UO2 samples immersed in mineral water at 90°C has been identified as magnesium silicate precipitated from the mineral water. Depth-profiling experiments showed that this magnesium silicate deposit was ca. 100–200 nm in thickness and revealed the formation of a thin film of UO3 between the bulk UO2 and the outer magnesium silicate deposit.

Quenched-in vacancies in single crystalline uranium monocarbide (UC)

Abstract Low-temperature electrical resistivity change due to quenched-in vacancies in single crystalline uranium monocarbide (UC) has been investigated. The formation energies for carbon and uranium vacancies are 1.1 ± 0.3 and 3.7 ± 0.4 eV, respectively. The isochronal annealing curves of quenched-in resistivity, obtained by resistivity measurements at room temperature, show two stages at about 600 and 1100 K, whereas those obtained by resistivity measurements at 77 K exhibit only the stage at 1100 K. The annealing stage at 1100 K is ascribed to the migration of uranium vacancies, of which the activation energy is evaluated as 2.1 ± 0.5 eV. It is found that the quenched-in resistivity depends on temperature; a quadratic temperature dependence is observed below 220 and 160 K for the quenched-in carbon and uranium vacancies, respectively.

Anomaly in magnetic susceptibility of crystalline uranium(IV) sulfate.

A new crystalline uranium (IV) sulfate has been prepared. Its magnetic susceptibility was measured from room temperature to 1.8 K and revealed broad maximum at 21.5 K. This anomaly was attempted to analyze by one-dimensional magnetic chain model with isotropic exchange interaction.

The preparation of UO 2 by fused salt electrolysis using UO 2 or UF 4 as starting material

Coarse crystalline uranium dioxide can be prepared by the electroreduction of UO 2Cl 2 or UO 2F 2 dissolved in fused chloride melts. The uranyl compounds are prepared in the melt by the treatment of UO 2 or UF 4 with a gas mixture of hydrogen chloride and oxygen. It is found that moisture accelerates the oxidation of the fluoride. Using the LiCl-KCl system, oxides with a composition ranging from UO 2.000 to UO 2.007 are obtained. The crystal form is affected by the degree of dryness of the melt and in the case of the LiCl-KCl-fluoride system also by the presence of chlorine in the melt. The presence of fluorides in the melt favourably influences the formation of cubeshaped UO 2 crystals. The oxides have densities varying between 10.85 and 10.98 (25° C).