Syntheses, Characterization, and Fundamental Studies of Novel Trivalent Lanthanide and Actinide Borates

The use of molten boric acid as a reactive flux in the synthesis of trivalent actinide borates has been explored and developed over the past several years. Throughout these investigations, several unexpected structures and results were obtained. It has been determined that the chemistries of trivalent plutonium, americium, curium, and californium within a borate matrix are completely different. This has lead to the first family of compounds containing the later actinides where the structures are neither isotypic to the lanthanides nor each other.

There are two main objectives of this work. The first is to further examine the basic chemistry and periodic trends between the lanthanide and the actinide elements. This has been achieved by functionalizing the f-element centers within these borates and by fine tuning the experimental conditions, which has led to two distinct families of the f-element borates. The second objective is to better understand the differences in bonding that exist between the two f-element series. This was achieved by Density Functional Theory (DFT) studies that have provided a more detailed examination of the f-elements which, to this point, has been shown to be the same in regards to structure and bonding analyses have often been contradictory. The DFT results also reveal real differences in bonding in the actinide elements. An understanding of these two objectives will aid in the development of structure-property relationships for advanced waste forms for the actinides present in nuclear waste, the environmental impacts of actinide borates, and an improved understanding of bonding in the f-elements to be exploited for chemical separations.

This dissertation is focused on the borate crystal chemistry of plutonium (Chapters 3, 5-7), americium (Chapters 3, 5, 7), curium (Chapters 4, 5, 7), californium (Chapter 11), and the lanthanides (Chapters 5 -10). While there are several advancements of more applied applications gained from this work, the most important academic and fundamental insight is that the chemistries of the later actinides and lanthanides can be vastly different and care must be taken when making assumptions about these highly underexplored actinides based on the chemistry of either the lanthanides or other actinides.