Dynamical properties of LiI-D2O: Ion diffusion and collective fluctuations

Analysis of NMR and conductivity data in lithium iodide monohydrate supports the idea that lithium diffusion is due to a small fraction (~10-4 at 250 K) of highly mobile ''defects.'' Furthermore, the spin-lattice relaxation times of 7Li indicate that the iodine cages around the immobile Li+'s, within several lattice spacings from a mobile defect, are influenced by its motion. The collective aspects of the ion dynamics are further investigated by photon correlation measurements, as a function of temperature, in a single deuterated crystal of lithium iodide monohydrate. The correlation function for the density fluctuations at optical wave vectors consists of two very distinct decay processes, each characterized by an average correlation time ??? and a parameter ? describing the width of the ? distribution. This is true for both the polarized and the depolarized scattering. A comparison between the correlation times ??? and the hopping times for a single particle shows that the density fluctuations are due to collective motion of clusters with size, presumably, comparable to that of the optical wavelength. A tentative interpretation is proposed which involves the structural relaxation of an orthorhombic phase into the high-temperature cubic form of LiI?D2O, as suggested by neutron-diffraction data.