Raman and infrared spectra of rubidium and caesium uranates(VI) and some problems assigning diuranate site symmetries

Abstract

The vibrational spectra of rubidium and caesium mono- and diuranates, M 2UO 4 and M 2U 2O 7 (M=Rb or Cs), have been measured in the infrared range, 4000–200 cm −1, and the Raman shift range of 1100–50 cm −1. The Raman spectra of these uranates are reported for the first time. The uranate ion site symmetries were assigned, based on the analysis of the spectra, as D 4h for Rb 2UO 4, D 2h for Cs 2UO 4. Assigning definite site symmetries for the diuranates presented some difficulties because it was not possible to do so unambiguously from spectral data but the likeliest for Rb 2U 2O 7 is C 2h and either C 2 or C s for Cs 2U 2O 7. The electronic spectra of molecules with a centre of symmetry are predicted to exhibit a greater temperature effect than acentric species. This is here shown experimentally, for the first time from diffuse reflectance spectra, by comparing the spectra of caesium mono- and diuranates at liquid nitrogen and room temperature; the much greater temperature effect of centrosymmetric over acentric species is clearly revealed by this technique. Crystallographic cell parameters of the uranates are reported, from X-ray powder diffraction studies.