Vibrational spectroscopic studies of tervalent hexa-aqua cations: Oriented single-crystal Raman spectra between 275 and 1 200 cm-1 of the caesium rhodium alums, CsRh(SO4)2·12H2O, CsRh(SeO4)2·12H2O, and CsRh(SO4)2·12D2O

Abstract

Oriented single-crystal Raman spectra of the caesium α-alums CsRh(SO4)2·12H2O, CsRh(SeO4)2·12H2O, and CsRh(SO4)2·12D2O have been recorded at or below 80 K. The external modes of water co-ordinated to the mono- and ter-valent cations as well as the internal modes of S(Se)O42- and [Rh{OH(D)2}6]3+ are found in the spectral region 275-1 200 cm-1. A full assignment of the spectra in this region has been completed and the results are in close agreement with the predictions of factor group analysis. This work provides the first full vibrational characterisation of the Raman-active modes of the sulphate α-alum lattice. The site and factor group splitting of the XO42- vibrational modes is independent..