Exploring Actinide Bonding and Crystal Chemistry in Acidic Media with Borate, Sulfate, Phosphate, and Arsenate Ligands

Herein we explore the vast coordination and crystal chemistry of the actinides by probing various ligand systems in acidic media. Unique solvothermal synthetic techniques were developed and implemented in order to synthesize new crystals yielding a variety of new structures containing the acidic solvent of choice. The main reaction medium used was oleum, or fuming sulfuric acid, which presented an uncommon avenue of study in the solid-state, given the lack of literature using these methods with actinide-bearing compounds. Additional research was done on the hydronium cation, a common cation in acidic aqueous systems, and the radiation stability of hydronium in uranium-bearing compounds. The first area of research explored the development of methods required to synthesize new compounds in oleum, SO3 enriched sulfuric acid. The use of 20% free SO3 H2SO4 presented various unique challenges when combining with the safety necessary in handling the actinides. Once established, these methods were executed in order to synthesize the first actinide borosulfate compounds. The first two actinide borosulfate compounds were both synthesized using solvothermal techniques with oleum and are both uranium-bearing compounds. Two out of the three unique bridging modes within borosulfate chemistry were targeted in these structures, the conventional borate-to-sulfate (B-O-S) and the unconventional borate-to-borate (B-O-B) bridging modes. The second area of research further expanded the synthetic methods of acidic media and the actinides by exploring with transuranic elements, neptunium and plutonium. These two elements have an extensive redox landscape and introduced more precautionary measures given the higher activity of the isotopes. Exploring neptunium and plutonium sulfate syntheses with oleum, concentration sulfuric acid, and H5[B(SO4)4] yielded some of the first anhydrous transuranic sulfates, as well as targeted an uncommon oxidation state sulfate, plutonium(III) sulfate. The third area of study investigated the environmentally relevant minerals, hydronium uranyl phosphate (HUP) and hydronium uranyl arsenate (HUAs). Hydronium, H3O+, is extremely prevalent in acidic aqueous solutions, but is far less common in solid-state compounds. The presence and placement of hydronium in HUP was analyzed with neutron powder diffraction and was compared to density functional perturbation theory calculations. Radiation stability of these low solubility compounds was also explored.