Hydrothermal transformations of hydrated cerium(IV) fluoride in NaOH or HF media. The first crystal structure in the cerium tetrafluoride hydrate family

Abstract

In order to synthesize new nanocrystalline compounds of cerium fluorides, the transformations of hydrated cerium(IV) fluoride with the composition CeF₄⋅H₂O under mild hydrothermal conditions were studied, in aqueous alkaline (1 M NaOH) or acidic (1 M HF) media. In alkaline medium, at 130 °С and 220 °C, hydrated cerium(IV) fluoride hydrolyses to form CeO₂, while in 1 M hydrofluoric acid at 130 °C, a mixture of crystalline Ce₃F₁₀⋅3H₂O and CeF₃ is formed. Hydrothermal treatment of cerium(IV) fluoride hydrate in 1 M HF at 220 °C yielded a new cerium(IV) fluoride hydrate (CeF₄)₃^.H₂O (Ce₃F₁₂^.H₂O). The new compound was studied by chemical analysis, powder and single crystal X-ray diffraction, scanning electron microscopy, ¹⁹F{¹H} and ¹H NMR, IR- and Raman spectroscopy. The noncentrosymmetric crystal structure of Ce₃F₁₂^.H₂O was found to be monoclinic (sp. gr. Cm, Z = 4) being isomorphic to uranium or neptunium tetrafluoride hydrates (U₃F₁₂^.H₂O or Np₃F₁₂^.H₂O). The structure of this compound exhibits a three-dimensional network of corner- and edge-shared [CeF₉], [CeF₇(OH₂)], [CeF₈(OH₂)] and [CeF₈] polyhedra. The influence of the cerium(IV) cations coordination sphere composition on the relative lengths of Ce-O bonds and F…HO hydrogen bonds discussed. The temperature behavior of the Ce₃F₁₂^.H₂O powder up to 500 °C was studied by thermogravimetry and differential scanning calorimetry.