S. Skanthakumar

A comparison of neptunyl(V) and neptunyl(VI) solution coordination: the stability of cation-cation interactions.

The solution coordination environments of pentavalent and hexavalent Np are studied by high-energy X-ray scattering. Np5+ and Np6+ both exist as the neptunyl moiety coordinated with five equatorial waters at Np-O distances of 2.46(2) and 2.37(2) A, respectively. NpO2(2+) also has a second coordination sphere of 6-10 waters at 4.37(3) A. The NpO2+ scattering is complicated by the presence of scattering at about 4.2 A that is attributed to Np-Np cation-cation interactions. The analysis of changing intensity of this peak as a function of Np concentration is used to determine a stability constant of Keq=0.74(9) M(-1) for the dimeric complex.

Structures and Energetics of Erbium Chloride Complexes in Aqueous Solution

A series of aqueous, constant ionic strength solutions containing 0.5 m Er is studied as a function of chloride/perchlorate ion concentration using high-energy X-ray scattering (HEXS) to probe the metal ion coordination environment. Perchlorate is seen to form an outer-sphere complex only in the end member (4.0 m perchlorate) of the series. Chloride ions are seen to bind as both inner- and outer-sphere complexes. A quantitative analysis of the scattering data is used to determine stability constants that depend on whether the complexation with chloride ions is assumed to add to or to replace the waters bound in the first coordination sphere. Published stability constants obtained from liquid-liquid extraction clarify the mechanism involved and, together with the HEXS data, present a consistent picture of the chemistry. The stability constants determined from the HEXS data, beta(1) = 0.38(8) and beta(2) = 0.014(9), are small, less than the available thermal energy in solution at room temperature. The combination of chemical separations and HEXS data for the analysis supports the argument that the complexes formed are real under the experimental conditions. Details of the structural correlations and interactions are discussed for applications including separations and environmental modeling.

Structural Correspondence between Uranyl Chloride Complexes in Solution and Their Stability Constants

Pair-distribution functions (PDF)s were obtained from high-energy X-ray scattering (HEXS) data on a series of uranyl solutions as a function of chloride ion concentration. Analyses reveal that chloride forms only inner-sphere complexes with the uranyl, replacing inner-sphere waters such that the total uranyl coordination number decreases from 4.7 waters at [Cl(-)] = 0 m to 4.4 (1.7 water and 2.7 Cl(-)) at [Cl(-)] = 6.8 m. Some of the second-coordination sphere waters reorient upon uranyl inner-sphere chloride complexation in order to hydrogen bond with the bound anion. Similar data obtained on a series of solutions maintained at constant ionic strength are used to confirm structural assignments through determining stability constants for the addition of chloride to uranyl and comparison with published values. The stability constants, β(1) = 1.5(10) m(-1), β(2) = 0.8(4) m(-2), and β(3) = 0.4(1) m(-3), obtained in a series of solutions with constant ionic strength of 5.3 m, are in reasonable agreement with previously published results determined by solvent extraction. The agreement of stability constants supports our peak assignments for the PDF and thus our structural model for uranyl chloride complexes in solution. Using coordination numbers and speciation determined here as a function of chloride ion concentration, the monochloro species is found to have four coordinating waters in the uranyl equatorial plane, the dichoro species is found to be an equilibrium of three and two coordinating waters, and the trichloro species has only a single water in the equatorial plane. These values correspond to total average coordination numbers of 5, 4.3, and 4 for the mono-, di-, and trichlorouranyl complexes. From the equilibrium value of the dichloro species, it can be further estimated that ΔG = -0.5 kcal/mol for the conversion of five to four coordinate species. Overall, the HEXS data support the assertion that uranyl chloride correlations do exist and the results are not simply the result of solvent-ion effects.

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.

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.

Magnetic properties of Dy in Pb2Sr2DyCu3O8

Abstract Superconductivity can be induced at high temperatures in Pb 2 Sr 2 RCu 3 O 8 (R, rare earth) by partially doping Ca 2+ for R 3+ . In order to understand the interplay between magnetism and superconductivity, the magnetic properties of the parent compounds, Pb 2 Sr 2 RCu 3 O 8, have been studied. The work presented here includes magnetic susceptibility and specific heat measurements on R=Dy and extends our previous studies on R=Ce, Pr, Tb, Ho and Er. Specific heat experiments suggest that the Dy ions order magnetically with an ordering temperature of 1.3 K. The magnetic susceptibility data are in good agreement with the susceptibility calculated using crystal-field parameters that are extrapolated from previous modeling of the R=Er and Ho analogs of this series.

The magnetic properties of Pr in the Pb2Sr2PrCu3O8+δ cuprate

Abstract Inelastic and elastic neutron scattering data on Pb 2 Sr 2 PrCu 3 O 8+ δ are presented. Magnetic ordering of the Pr moments is found to be analogous to the Tb analogue, interpreted with two different wave vectors k =0 0 0 and 0 0 1/2. A molecular field-induced shift is observed in the low-energy inelastic neutron scattering (INS) data when going through the Neel temperature. This shift is quantitatively accounted for by the magnetic order of the Pr sublattice. The crystalline-electric-field (CEF) potential has been determined from the observed magnetic excitations and is compared with those of Pr in other cuprates. These results underline the relevance of the quasi-triplet ground state splitting for T c suppression by Pr in the cuprate superconductors.

Electronic and magnetic properties of Cm in Pb2Sr2Cm1-xCaxCu3O8 (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.

Electronic and magnetic properties of Cm in Pb{sub 2}Sr{sub 2}Cm{sub 1-x}Ca{sub x}Cu{sub 3}O{sub 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.