Uranyl Chloride Transformations under Atmospheric Conditions

Abstract

While the speciation of the uranyl cation (UO22+) in chloride solutions is fairly well understood, spectroscopic characterization and investigation of the analogous solid-phase species are lacking. We report here the transformation of solid uranyl chloride hydrate complexes in air starting from UO2Cl2(H2O); this transition is characterized by the formation of five successive solid phases: (UO2)4O2Cl4(H2O)6·4H2O, two polymorphs of UO2Cl2(H2O)3, (UO2)4O2Cl4(H2O)6·2H2O, and (UO2)2(OH)2Cl2(H2O)4. Each phase in this system produces distinct morphologies and unique spectroscopic signatures. Extensive hydrogen-bonding networks stabilize these complexes, which lead to sensitivity to atmospheric water and a deliquescent behavior that drives some phase transformations. Lengthening of the uranyl bond and bending of this structural unit as a result of coordination to μ2-OH and μ3-O hydrolytic bridging ligands are characterized using single-crystal X-ray diffraction, Raman spectroscopy, and density functional theory modeling with good agreement between techniques. A dimeric complex, (UO2)2(OH)2Cl2(H2O)4, represents the stable, terminal phase after months of aging in air, suggesting a structural stability of polynuclear uranyl chloride complexes over monomeric solids. Characterization of these compounds supports the identity of monomeric and polynuclear aqueous uranyl species through investigation of these molecular solid analogs with greater implications for environmental speciation of the uranyl cation.