Raman Spectroscopic Study of the Uranyl Tricarbonate Mineral Liebigite

Raman spectroscopy at 298 and 77 K has been employed to study the structure of the uranyl tricarbonates liebigite {Ca2[UO2(CO3)3]11H2O}. The spectra of the samples are sample dependent and significant differences in the Raman spectra are obtained upon cooling to 77 K. It is proposed that the mineral undergoes a phase change upon cooling. Significant changes in the Raman spectra are observed in the hydroxyl stretching region and in the low wavenumber region. Two Raman bands are observed at 3468 and 3528 cm-1 showing the non-equivalence of the water units in the structure. Obtaining spectra at 77 K enables well resolved bands at 3530, 3501, 3482, 3463, 3443, 3405 and 3364 cm-1. Two well resolved bands at 1087 and 1073 cm-1 in the 298 K spectra become bands at 1093, 1076 and 1008 cm-1 in the 77 K spectrum proving the non-equivalence of the carbonate units. This non-equivalence is reflected in the observation of multiple bands in the carbonate bending regions. The (UO2)2+ units are characterised by the antisymmetric stretching vibrations at 902, 885 and 873 cm-1 and by the ν2 bending modes at 248 cm-1. Two bands near 820 cm-1 may be assigned to the ￮1 (UO2)2+ symmetric stretching vibrations, however, a coincidence of these vibrations with the ￮2 (CO3)2- out-of plane bending vibrations cannot be excluded. The use of Raman spectroscopy enables a better definition of the vibrational modes of liebigite and shows changes in the molecular structure upon reaching 77 K.