C. W. Williams

Analysis of the crystal‐field spectra of the actinide tetrafluorides. II. AmF4, CmF4, Cm4+:CeF4, and Bk4+:CeF4

We report a systematic analysis of the crystal‐field spectra of four fluoride compounds containing tetravalent actinide ions. The first part of this work [J. Chem. Phys. 95, 7194 (1991)] provided interpretation of the absorption spectra of UF4, NpF4, and PuF4. To extend our analysis to heavier elements of the series, low‐temperature absorption spectra of AmF4 and CmF4, and site selective laser‐induced emission and excitation spectra of Cm4+:CeF4 and Bk4+:CeF4 were obtained. A model energy level calculation was found to be in good agreement with the experimental results. It is shown that the crystal‐field interaction in combination with spin–orbital coupling results in significant J mixing in the excited states, but ground state wave functions are still relatively pure in J character for the tetravalent actinide ions Am4+, Cm4+, and Bk4+. Trends in the parameters of the effective operator Hamiltonian are compared with those of a Hartree–Fock free‐ion model. Interpretation of the ground‐state splitting of t...

Preparation, stability, and structural characterization of plutonium(VII) in alkaline aqueous solution.

A freshly prepared solution of Pu(VI) in 2 M NaOH was oxidized to Pu(VII), via ozonolysis, while simultaneously collecting X-ray absorption spectra. Analyses of the XANES (X-ray absorption near edge structure) and EXAFS (extended X-ray absorption fine structure) data, acquired throughout the in situ experiments, show a dioxo coordination environment for Pu(VI), PuO(2)(2+), typical for it and the hexavalent actinyl species of U and Np, and its evolution into a tetraoxo-coordination environment for Pu(VII), PuO(4)(-), like that known for Np(VII). The EXAFS data provide average Pu-O distances of 1.79(1) and 1.88(1) Å, respectively. The second coordination shells, also fit as O atoms, provide Pu-O distances of 2.29-2.32 Å that are independent of the Pu oxidation state. The coordination numbers for the distant O atoms in sums with those for the nearest O atoms are consistent with 6-O environments for both Pu(VI) and Pu(VII) ions in accordance with their previously proposed speciation as [Pu(VI)O(2)(OH)(4)](2-) and [Pu(VII)O(4)(OH)(2)](3-), respectively. This solution speciation accounts precisely for the Pu(VI) and Pu(VII) coordination environments reported in various solid state structures. The Pu(VII) tetraoxo-dihydroxo anion was found to have a half-life of 3.7 h. Its instability is attributed to spontaneous reduction to Pu(VI) and not to a measurable extent of disproportionation. We found no direct evidence for Pu(VIII) in the X-ray data and, furthermore, the stoichiometry of the oxidation of Cr(III) by Pu is consistent with that expected for a valence-pure Pu(VII) preparation by ozonation and, in turn, stoichiometrically equivalent to the established Np(VII)/Cr(III) redox reaction.

Studies of local structure of Cm3+ in borosilicate glass using laser and x-ray spectroscopic methods and computational modeling

The local environment of Cm3+ in a borosilicate glass has been probed by a combination of laser spectroscopy, structural modeling, and extended x-ray absorption fine structure (EXAFS) spectroscopy. The Stark splitting for the Cm f–f state transitions is significantly larger than the inhomogeneous line broadening that results from the disordered environment. As a result, the Cm optical spectrum can be fit using an effective operator Hamiltonian to obtain a set of crystal-field parameters. The fitting procedure, which requires the use of a descent-in-symmetry approach, provides a set of parameters for a best fit within tetragonal symmetry. These parameters are then linked to the local environment of Cm through exchange-charge modeling (ECM) of crystal field interactions. Cm in our borosilicate glass is best modeled with six oxygen ions with approximately tetragonal symmetry, and at an average distance of 2.31 (3) A. The results of crystal-field modeling are supported by EXAFS results.

Uranyl fluoride luminescence in acidic aqueous solutions

Abstract Luminescence emission spectra and decay rates are reported for uranyl species in acidic aqueous solutions containing HF or added NaF. The longest luminescence lifetime, 0.269±0.006 ms, was observed from uranyl in 1 M HF+1 M HClO4 at 296 K and decreased with increasing temperature. Based on a luminescence dynamics model that assumes equilibrium among electronically excited uranyl fluoride species and free fluoride ion, this long lived uranyl luminescence in aqueous solution is attributed primarily to UO2F2. Studies on the effect of added LiNO3 or Na2WO4·2H2O showed relatively weak quenching of uranyl fluoride luminescence which suggests that high sensitivity determination of the UF6 content of WF6 gas should be feasible via uranyl luminescence analysis of hydrolyzed gas samples of impure WF6.

The effect of f-ion valence on superconductivity in the series Pb2Sr2RCu3O8 (R=Ce, Pr, Tb and Am)

Abstract The magnetic properties and valences of the rare earth ions are probed in the superconducting series Pb2Sr2R1−xCaxCu3O8 (R=Ce, Pr, Tb and Am). The R=Pr and Tb samples are superconducting for x≈0.5 below 61 and 72 K, respectively, whereas the Ce and Am samples could not be made superconducting for any single phase sample, irrespective of x. X-ray absorption near edge spectroscopy (XANES), used to probe the valence of R, reveals that Pr and Tb are trivalent whereas the Ce and Am are tetravalent. This direct correlation of R valence and the observation of superconductivity is discussed.

Structure and charge transfer dynamics of uranyl ions in boron oxide and borosilicate glasses

Laser spectroscopic experiments, molecular dynamics simulation, and charge transfer-lattice interaction modeling have been conducted for studying the electronic and structural properties of the uranyl ion UO22+ in boron oxide and borosilicate glasses. The charge transfer electronic and vibrational energy levels for uranyl ions in the glass matrices were obtained from laser excitation and fluorescence spectra of UO22+. A model structure for uranyl ions in the glass matrices was established using the method of molecular dynamics (MD) simulation in comparison with the results of extended x-ray absorption fine structure (EXAFS) for U6+ ions in the glasses we studied. The formation and stabilization of uranyl clusters in glass matrices are interpreted in terms of charge transfer-lattice interactions on the basis of self-consistent charge transfer accompanied by lattice distortion. The latter is in the framework of the simultaneous action of pseudo-Jahn-Teller and pseudo-Jahn-Teller analog effects on charge transfers between oxygen and uranium ions.

Formation of UO2(BO3)4 clusters in B2O3 glass due to charge transfer and vibronic effects

Laser-induced excitation and fluorescence spectra and X-ray absorption fine structure (EXAFS) of the U{sup 6+} L{sub 3} edge (2p-5d) transitions indicate that uranyl ions in B{sub 2}O{sub 3} glass have ordered local structure. Based on molecular dynamics (MD) simulation, four oxygen ions bind with each linear uranyl in the equatorial plane to form a stable crystalline UO{sub 6} cluster that binds with four groups of BO{sub 2}. A charge transfer-lattice interaction model for the UO{sub 6} cluster formation in B{sub 2}O{sub 3} glass is developed to explain the strong local lattice ordering and extraordinary stability of this cluster. The model is based on self-consistent oxygen-oxygen and oxygen-uranium charge transfer accompanied by lattice distortion both in the framework of pseudo-Jahn-Teller scheme.

Electronic and magnetic properties of Cm in Pb 2 Sr 2 Cm 1 − x Ca x Cu 3 O 8 ( x = 0.0 and 0.5)

A combination of x-ray diffraction, x-ray absorption spectroscopy, and magnetic susceptibility measurements have been used to study the physical properties of Pb{sub 2}Sr{sub 2}Cm{sub 1-x}Ca{sub x}Cu{sub 3}O{sub 8} (x=0.0,0.5). These Cm compounds are isostructural with the superconducting members of the Pb{sub 2}Sr{sub 2}R{sub 1-x}Ca{sub x}Cu{sub 3}O{sub 8} (R denotes rare earth) series. X-ray absorption and magnetic susceptibility data clearly indicate that the Cm is trivalent in this compound, consistent with the superconducting members of this series. However, Pb{sub 2}Sr{sub 2}Cm{sub 0.5}Ca{sub 0.5}Cu{sub 3}O{sub 8} is not superconducting. Furthermore, the Cm spins are found to magnetically order in the parent compound Pb{sub 2}Sr{sub 2}CmCu{sub 3}O{sub 8} with an ordering temperature of 18 K. Suppression of superconductivity is observed together with high-magnetic ordering temperatures in other high-T{sub c} related compounds synthesized with Cm. The results determined here are put into context with these previous observations.

The Influence of Desulfovibrio Desulfuricans on Neptunium Chemistry

The role of biotic Np(V) reduction is studied in light of its potential role in 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 and X-ray absorption near edge structure (XANES) were used to determine that Np(V) is almost quantitatively reduced in all three samples and that the precipitate is an amorphous Np(IV) species. These results demonstrate that the reduction of Np is independent of Desulfovibrio desulfuricans. The underlying chemistry associated with these results is discussed.

Preparation, chemical reactions, and some physical properties of neptunium pentafluoride

Abstract The preparation of NpF 5 by reacting a solution containing NpF 6 − with the Lewis acid BF 3 in anhydrous HF is reported. The purity of NpF 5 was established by its Mossbauer spectrum, which showed only the presence of Np 5+ . Thermal decomposition and some chemical reactions of NpF 5 are described. Debye-Scherrer powder data from NpF 5 appear similar to those obtained from α-UF 5 , indexing as tetragonal with cell constants a = 6.5358(18) A and c = 4.4562(14) A . Photochemical irradiation of NpF 6 produced an amorphous material believed, but not confirmed, to be NpF 5 .