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各类配体体系，系统考察锕系元素（actinides）丰富的配位化学与晶体化学特性。我们开发并应用了独特的溶剂热合成技术，以合成含目标酸性溶剂的新型晶体，进而获得一系列全新结构。本研究选用的主要反应介质为发烟硫酸（oleum），鉴于当前鲜有采用此类方法开展含锕系元素化合物研究的文献报道，该介质为固态化学研究开辟了一条罕见的路径。此外，本研究还针对酸性水相体系中常见的水合氢离子（hydronium cation），以及含铀化合物中水合氢离子的辐射稳定性展开了相关研究。
第一研究方向聚焦于开发在富含三氧化硫的硫酸（即发烟硫酸）中合成新型化合物的方法。当结合操作锕系元素所需的安全规范时，使用游离三氧化硫含量为20%的硫酸会面临诸多独特挑战。上述方法确立后，我们利用其合成了首批锕系元素硼硫酸盐化合物。首批两种锕系元素硼硫酸盐化合物均采用发烟硫酸溶剂热技术合成，且均为含铀化合物。本研究针对硼硫酸盐化学中的三类独特桥联模式中的两类开展了结构构建，分别为常规的硼酸盐-硫酸盐（B-O-S）桥联模式与非常规的硼酸盐-硼酸盐（B-O-B）桥联模式。
第二研究方向进一步拓展了酸性介质与锕系元素的合成方法，选取超铀元素（transuranic elements）镎（neptunium）与钚（plutonium）开展研究。这两种元素拥有复杂多样的氧化还原态势，且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。通过使用发烟硫酸、浓硫酸与四硫酸合硼酸氢（H5[B(SO4)4]）开展镎与钚的硫酸盐合成研究，我们获得了首批无水超铀硫酸盐，同时成功合成了罕见氧化态的目标硫酸盐——硫酸钚(III)。
第三研究方向聚焦于与环境相关的矿物：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十分罕见。本研究通过中子粉末衍射分析了HUP中水合氢离子的存在形式与排布位置，并将实验结果与密度泛函微扰理论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。