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.本研究通过探究酸性介质中的多种配体体系，系统考察锕系元素（actinides）丰富的配位与晶体化学特性。本研究开发并应用了独特的溶剂热（solvothermal）合成技术，以制备含目标酸性溶剂的新型晶体，进而获得一系列全新结构的化合物。本研究采用的主要反应介质为发烟硫酸（oleum），鉴于目前鲜有针对含锕系化合物使用该类方法的相关文献报道，这一体系为固态化学研究提供了一条罕见的研究路径。此外，本研究还针对酸性水溶液体系中常见的水合氢离子阳离子（hydronium cation，H₃O⁺），以及含铀化合物中水合氢离子的辐射稳定性展开了研究。 第一部分研究聚焦于发烟硫酸（即三氧化硫（SO₃）富集的硫酸）中新型化合物的合成方法开发。当结合锕系元素操作所需的安全规范时，使用含20%游离三氧化硫的硫酸体系会面临诸多独特挑战。待方法体系建立后，本研究利用该路线合成了首例含锕系元素的硼硫酸盐（borosulfate）化合物。首例两种含锕系元素的硼硫酸盐化合物均通过发烟硫酸溶剂热技术合成，且均为含铀化合物。该类结构靶向了硼硫酸盐化学中三种独特桥联模式中的两种：常规的硼酸盐-硫酸盐（B-O-S）桥联模式，以及非常规的硼酸盐-硼酸盐（B-O-B）桥联模式。 第二部分研究以超铀元素（transuranic elements）镎（neptunium，Np）和钚（plutonium，Pu）为研究对象，进一步拓展了酸性介质中锕系元素的合成方法体系。这两种元素具有丰富的氧化还原化学行为，且由于其同位素具有更高的放射性活度，因此需要采取更多的防护措施。通过发烟硫酸、浓硫酸以及五氢四(硫酸根合)硼酸（H₅[B(SO₄)₄]）为反应介质，开展镎和钚的硫酸盐合成研究，本研究成功制备了首批无水超铀硫酸盐化合物，同时靶向合成了一种罕见氧化态的硫酸盐——三价钚硫酸盐（plutonium(III) sulfate）。 第三部分研究针对与环境相关的矿物展开：水合氢离子铀酰磷酸盐（hydronium uranyl phosphate，HUP）与水合氢离子铀酰砷酸盐（hydronium uranyl arsenate，HUAs）。水合氢离子（H₃O⁺）在酸性水溶液中极为常见，但在固态化合物中却较为罕见。本研究通过中子粉末衍射技术分析了HUP中水合氢离子的存在状态与排布位置，并将实验结果与密度泛函微扰理论（density functional perturbation theory，DFPT）计算结果进行了对比。此外，本研究还考察了这类低溶解度化合物的辐射稳定性。