Developments in Actinide Solution- and Solid-State Chemistry

A recent drive to explore the fundamental properties of later actinide elements is culminated by this research which targets the structural chemistry and electronic behavior of berkelium in the solid-state. Experimental evidence collected for materials containing plutonium, americium, curium, and californium have stirred interest in revising models explaining bonding character in the actinides. Covalency in chemical bonding is linked to the differences in crystalline structure which these elements possess upon reaction under identical experimental conditions. Moderate half-lives and dangerous decay energies provide obstacles for actinide research in terms of safety and handling, but production of milligram quantities of these elements at HFIR in ORNL overcomes the major concern of sufficient yield in material. The first four single crystal berkelium materials to exist were developed in solvothermal [Bk(HDPA)₃·nH₂O], flux {Bk[B₆O₈(OH)₅]}, and hydrothermal synthesis reactions [Bk(IO₃)₃ and Bk(IO₃)₄]. In a series of experimental and quantum mechanical techniques, the bonding dynamics of berkelium could be related to its actinide neighbors, curium and californium, as well as its lanthanide isoelectronic analog, terbium. The results support the introduction of hybridized 5f6d bonding orbitals in the trivalent berkelium materials, something which is absent in corresponding curium and terbium compounds, but present in those with californium. Quantum mechanical calculations expose this observation and isotypic structure-types between berkelium and californium, and not curium or terbium, corroborate this finding. However, splitting of the ground state energy by spin-orbit coupling is an order of magnitude smaller in berkelium, like curium, compared to californium. Therefore, conclusions of berkelium’s bonding nature can be resolved to show structural chemistry akin to californium, but electronic behavior similar to curium. The first "formally" tetravalent berkelium material, Bk(IO₃)₄, displayed spectroscopic behavior of trivalent berkelium, a property known as mixed-valency. These bulk-scale experiments expose the unique fundamental chemistry of berkelium, an endeavor that is difficult to achieve when considering the half-lives of and protection against the decay energies of ²⁴⁹Bk and ²⁴⁹Cf, the daughter of ²⁴⁹Bk. Disruption of typical electronic behavior in the earlier actinides was sought by slow evaporation and solid-state characterization experiments. A typical reductant used in nuclear separations to maintain Pu[superscript IV] and Np[superscript IV] and leave U[superscript VI], FHA was used as a complexing ligand in crystallization experiments with U[superscript VI] and Pu[superscript IV]. Crystallization of UO₂(FHA)₂ produced red block crystals, where the atypical red color from yellow-green is a result of distortions in the absorption of this material due to weakening of the O=U=O bond by 6.5° from linear. Pu₂(FHA)₈ crystallizes as a discrete dimeric complex, with inversion symmetry that may be a key to observing the nuclear moment of plutonium using ²³⁹Pu-NMR. Reversing the role of FHA from reductant to ligand provided evidence of strong π donation in actinide complexes that suggest its application in uranium seawater extraction. Finally, collaborative work between CEMRC and INE aims at producing a model that explains actinide solubility data in high ionic strength borate media. Undersaturation experiments with Np[superscript V] represent the final measurements needed to effectively model its solubility and speciation behavior in high ionic strength brine solutions of similar consistency to that of natural geological repositories. Preliminary analysis of Pu[superscript VI] in varying borate solutions expose two different species of plutonium borate, a result of the ability for borate to polymerize in aqueous media. This dissertation is the collective work of three projects in two different topic areas. The work is presented in chronological order of experiment and publication. Chapter four and five review the effects in structure and bonding in UO₂(FHA)₂ and Pu₂(FHA)₈ due to the coordination by FHA. Chapters six and seven expose the fundamental chemistries of berkelium and its qualities in relation with its neighbors in the actinide series. Speciation and solubility data for Np[superscript V] and Pu[superscript VI] borate systems are discussed in chapter eight and nine. Collectively, this work explains unique phenomena in higher valent actinide complexes, the distinct behavior of berkelium in sold-state, and the migration of Np[superscript V] and Pu[superscript VI].