Similarities between the Coordination of Actinide Ions in Solution and the Structure of their Related Crystalline Phases
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Similarities between the Coordination of Actinide Ions in Solution and the Structure of their Related Crystalline Phases L. Soderholm1,2, S. Skanthakumar1, Peter C. Burns1,2 and Tori Forbes2 1 Chemistry Division, Argonne National Laboratory, Argonne, IL 60439 2 Department of Civil Engineering and Geological Sciences, University of Notre Dame, Notre Dame IN 46556
ABSTRACT 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.
INTRODUCTION A fundamental understanding of an element’s structural chemistry is incomplete until its preferred ligand arrangements are known in both solution and the solid state. A knowledge of coordinating-ligand preferences, coordination numbers, bond distances and ligand geometries under a variety of conditions are necessary for a predictive understanding of the chemistry of an element. Whereas there is a relatively good understanding of the coordination geometries of many of the chemical elements, this knowledge does not extend into the actinide series. A good basis for solid-state structural chemistry exists for uranyl compounds [1, 2] and to a lesser degree for lower-valent U and Th compounds [3], which has been built upon a groundwork of known mineral phases. In contrast, the crystal chemistry of transuranic elements remains largely unknown and unexplored. Although no geologic precedent is available, indications from recent structural studies on neptunyl(V) compounds reveal a generally different coordination environment of NpO2+ over its UO22+ analog [4-7]. Although there is very little crystal chemical work on lower-valent transuranic compounds [8-11], the tendency is to follow trends established in work with the lanthanides. There is relatively little known about the structural chemistry of actinide ions in solution. This situation arises from both the inherent complexity of actinide solution chemistry and the lack of a good structural probe, a situation shared across the Periodic Table. Hydrolysis, radiolysis and redox reactions often contribute to the presence of multiple species in solution. Extended x-ray absorption near-edge structure (EXAFS) spectroscopy [12] has advanced the understanding of actinide speciation, particularly under conditions relevant to the environment and geochemistry [13]. Unfortunately, many of the important questions about actinide coordination in solution and adsorbed onto surfaces remain unanswered because of the inherent uncertainties in the coordination numbers obtained by EXAFS and the difficulties encountered in probing distances longer than
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