Dynamical properties of LiI D2O II: Vibrational modes and disordering effects

The vibrational dynamics of ?-phase lithium iodide monodeuterate (LID) is investigated by Raman scattering as a function of temperature. A simple model is presented in order to explain the linkage between the D2O reorientation and the Li+ hopping motion in the superionic ? form. The internal O--D stretching and D2O bending regions are analyzed, within this model, by a suitable deconvolution of the symmetric lines. Spectral contributions that might originate from two possible polarization states of the D2O molecule are discovered. Raman spectra of the melt, both in the polarized (VV) and depolarized (VH) geometries, are also presented. The experimental data reveal that the melt, in spite of stronger anharmonicity effects, exhibits the same local order which is found in the ? phase. Furthermore, a measurement performed at a fixed temperature (T=-70 °C) as a function of time shows a dependence of the spectral features on time, which confirms the existence of a structural phase transition towards an orthorhombic ? form, as also suggested by neutron diffraction data. The low-frequency translational region shows the characteristic broad features of a density of vibrational states both in the ? phase and in the melt, thus confirming the highly cooperative nature of the dynamics of the system.