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.

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.

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.

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.

X-ray absorption data, obtained from Np(V) in two environmentally-related samples, show evidence of significant metal-ion reduction in the synchrotron beam. The data are presented and possible sources of reduction are discussed.

The effect of Am doping on the physical properties of three superconducting systems (RBa2Cu3O7, Pb2Sr2R1−xCaxCu3O8 and R2−xMxCuO4) have been studied using x-ray diffraction, x-ray absorption spectroscopy (XAS) and magnetic susceptibility experiments. Am is incorporated into all three systems as the tetravalent ion and superconductivity is not observed in any of the resultant compounds. The absence of superconductivity in Pb2Sr2Am0.5Ca0.5Cu3O8 is attributed to the transfer of charge from Am to the Cu-O planes. Systems that cannot incorporate a tetravalent ion, notably AmBa2Cu3O7 and Am2CuO4, do not form. Although Pr1.85Am0.15CuO4 forms with similar structure to Pr1.85Ce0.15CuO4, and Am like Ce is tetravalent, it is not superconducting. We argue that the absence of superconductivity in the Am doped Pr compound it is due to the hybridization of radially-extended f orbitals on the magnetic Am4+ ion with the Cu-O band states.

Insight into the factors that control the formation and size of heavy metal phosphate nanophases in vitreous silica has been gained by combining conventional and anomalous small angle x-ray scattering studies with powder x-ray diffraction and laser-induced fluorescence investigations. Europium, thorium, and uranyl ions were sorbed from aqueous solutions into a chemically functionalized porous silica (termed Diphosil). Aliquots of those samples were heated to a series of temperatures that spanned the pore collapse point. Loading with trivalent europium ions resulted in production of nanophases whose size corresponded to the average number of metal ions per pore. Thorium or uranyl ions resulted in retention of porosity to higher temperature with eventual pore collapse that evidently resulted in formation of nanophases whose size exceeded that determinable under the experimental conditions.

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.