Layer-structured uranyl-oxide hydroxy-hydrates with Pr(iii) and Tb(iii) ions: hydroxyl to oxo transition driven by interlayer cations.

We report the hydrothermal synthesis and characterization of two uranyl-oxide hydroxy-hydrate compounds with Pr(iii) (U-Pr) and Tb(iii) (U-Tb) ions prepared via direct hydrothermal reactions of lanthanide (Ln = Pr or Tb) ions with a uranyl-oxide hydroxy-hydrate phase, schoepite. Both compounds U-Pr and U-Tb show thin plate morphologies with atomic ratios of 2 (U : Pr) and 6 (U : Tb) and have been characterized by multiple techniques. The layered structures with interlayer hydrated Pr(iii) or Tb(iii) ions formed via uranyl-Pr/Tb interactions have been confirmed by synchrotron single crystal X-ray diffraction studies. In addition, the evolution of the uranyl oxide hydroxide layers and anion topologies upon increasing the concentration of interlayer cations by using different U : Ln (Ln = Pr or Tb) ratios has been discussed. The success in the preparation and characterization of compounds U-Pr and U-Tb with different U : Ln (Ln = Pr or Tb) ratios highlights the flexibility of the uranyl oxide hydroxide layers with respect to the incorporation of interlayer cations via a gradual hydroxyl to oxo transition. The study has direct implications in regard to the natural weathering of uraninite mineral and the alteration of spent nuclear fuels during the long-term geological disposal.