Raman studies of the vibrational dynamics and phase transitions in uranyl nitrate hexahydrate

The temperature‐dependent Raman spectra of crystalline uranyl nitrate hexahydrate, UO 2 (NO 3 ) 2 · 6H 2 O, are reported in the temperature range 15‐290 K. A group theoretically consistent assignment of the bands in terms of UO   2 2+ , NO 3 − and H 2 O vibrations is proposed. The temperature‐dependent studies of thermosensitive bands associated with these entities were used to study the previously reported phase transition at 140 ± 5 K and to detect another phase transition at 190 ± 5 K. The latter transition shows up prominently in the temperature‐dependent fluorescence studies and is also accompanied by a sharp increase in the Rayleigh scattering. The transition at 190 ± 5 K is triggered by the dynamics of H 2 O‐II and UO   2 2+whereas that at 140 ± 5 K is associated with the dynamics of H 2 O‐II/NO 3 − . Both the transitions are of second order.