VIBRATIONAL DYNAMICS AND PHASE-TRANSITION IN URANYL-NITRATE DIHYDRATE (DIDEUTERATE), UO2(NO3)2.2H2O (2D2O) THROUGH RAMAN-SPECTROSCOPY

Raman spectra of crystalline UO2(NO3)2.2H2O and its isotopic derivative UO2(NO3)2.2D2O have been investigated in the frequency range 20-3700 cm-1 from 300 to 90 K. An assignment of the bands in terms of UO22+, NO3-, and H2O vibrations has been proposed in the internal and external mode regions. The temperature evolution of the Raman spectra indicates a structural phase transition in UO2(NO3)2.2H2O at 180 K. The transition has a large critical region and is associated with the strengthening of the hydrogen bonded network. The strengthening of hydrogen bonds distorts the geometries of entities bonded to it, which is mainly reflected in the temperature evolution of water stretching and librational modes. The changes in the crystal structure at 180 K are also reflected through the changes in the frequencies of the nu-2 modes of nitrato (O-N-O2) groups. Some lattice modes of the entities associated in the hydrogen bonded network also corroborate the changes in the crystal structure at 180 K.