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

Abstract

To develop the structural chemistry of radium, the halide compounds RaX 2 •H 2 O and RaX 2 •2H 2 O (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 Ra 2+ chlorides differ from their Ba 2+ analogues. For MCl 2 •H 2 O (M 2+ = Ba 2+ , Ra 2+ ), the variance between the metal coordination environments manifests as a small, local distortion that becomes more apparent in the extended structure. However, differences between RaCl 2 •2H 2 O and BaCl 2 •2H 2 O are more pronounced with a 10coordinate Ra 2+ cation being observed instead of a nine-coordinate Ba 2+ in BaCl 2 •2H 2 O. RaBr 2 •nH 2 O (n = 1 or 2) are isomorphous with the Ba 2+ analogues. Raman spectroscopy was used as an additional probe of these compounds and reveals substantial shifts and different vibrational modes between RaX 2 •H 2 O and RaX 2 •2H 2 O compared to BaX 2 •2H 2 O.