Nmr relaxation and line shape study on Li+ diffusion in nanocrystalline layer-structured LixTiS2

Temperature and frequency dependent Li-7 spin-lattice relaxation rate measu rements on the layer-structured two-dimensional ion conductor LixTiS2 in di fferent order states were carried out in the laboratory frame (T-1(-1)) and in the rotating frame (T-1 rho(-1)). The activation energies for individua l ion hopping, as obtained from these measurements, are about 0.19eV for th e polycrystalline, 0.16eV for the nanocrystalline, and 0.07eV for the amorp hous material. The frequency dependence of T-1(-1) is sublinear for both di sordered modifications. The NMR central transition lines of the nanocrystal line material decompose into a narrow and a broad component in the course o f motional narrowing. The relative intensity of the narrow component corres ponding to the fraction of highly mobile Li ions increases gradually with t emperature, reaching a limiting value of 50% at high temperatures. Hence, w e conclude that the interfacial regions are not structurally homogeneous an d comprise about half of the atoms of the sample. Contrary to three-dimensi onal nanocrystals, diffusion in the two-dimensional nanocrystalline materia l takes place on the grain surfaces rather than within an amorphous interfa ce medium. (C) 1999 Acta Metallurgica Inc.