Deviation between the Structural Chemistry of Barium and Radium Halides: Synthesis and Characterization of RaX2·H2O and RaX2·2H2O (X– = Cl– and Br–)

To develop the structural chemistry of radium, the halide compounds RaX₂·H₂O and RaX₂·2H₂O (X^– = Cl^– and Br^–) have been synthesized and characterized and serve as benchmarks for comparisons with more complex compounds in the future. In contrast with historic reports on the structural chemistry of radium, the Ra²⁺ chlorides differ from their Ba²⁺ analogues. For MCl₂·H₂O (M²⁺ = Ba²⁺, Ra²⁺), the variance between the metal coordination environments manifests as a small, local distortion that becomes more apparent in the extended structure. However, differences between RaCl₂·2H₂O and BaCl₂·2H₂O are more pronounced with a 10-coordinate Ra²⁺ cation being observed instead of a nine-coordinate Ba²⁺ in BaCl₂·2H₂O. RaBr₂·nH₂O (n = 1 or 2) are isomorphous with the Ba²⁺ analogues. Raman spectroscopy was used as an additional probe of these compounds and reveals substantial shifts and different vibrational modes between RaX₂·H₂O and RaX₂·2H₂O compared to BaX₂·2H₂O.