Ultraslow ion motions in solids. NMR-spectroscopic studies on lithium-ion conductors

The dynamics and geometry of ultraslow Li motions - with jump rates from the kHz down to the sub-Hz range - were studied in a series of crystalline and amorphous ionic conductors by 7Li and 6Li multiple-time stimulated-echo (spin-alignment) NMR techniques. Polycrystalline LixTiS2, Li4SiO4, amorphous LiNbO3 and (Li, Na) niobium silicate glasses as well as single-crystalline Li3N served as model substances for the precise investigation fundamental aspects of ionic diffusion by two-time stimulated echo NMR spectroscopy. The spin-alignment NMR technique, which is based on the Jeener-Broekaert three-pulse sequence, takes advantage of the interaction between the quadrupole moment of the nucleus and the electric charge distribution in the neighbourhood of the nuclear site. The hopping induced temporal change of the quadrupolar frequency of a nucleus jumping between inequivalent sites with different electrical field gradients is used to trace ultraslow jump processes directly. For instance, the two-dimensional Li diffusion process in layered Li0.7TiS2 was extensively probed over a dynamic range of about ten orders of magnitude. Jump rates range between 1 x 10-1 s-1 and 7.8 x 108 s-1 (148-510 K). Whereas from recording diffusion induced spin-lattice relaxation rate maxima very fast Li jump rates in the GHz to kHz range can be probed, ultraslow Li jumps in the kHz to sub-Hz range were detected directly by recording mixing time dependent spin-alignment echoes as a function of temperature. Up to now such slow Li jump processes were not accessible by conventional NMR techniques. Moreover, the analysis of final state amplitudes of the single-particle correlation functions provides unique information to elucidate the Li diffusion mechanism in hexagonal Li0.7TiS2. Beside an extensive investigation of slow Li dynamics in amorphous LiNbO3, spin-alignment echo NMR was used for the first time to characterize the mixed cation effect in a glassy system like (Li, Na) niobium silicate. Furthermore, in Li4SiO4 extremely slow cation exchange processes among different crystallographical Li sites were detected directly by 7Li spin-alignment NMR. The results demonstrate that spin-alignment echo NMR is a time-saving alternative for the study of slow cation exchange processes via 6Li 2D exchange NMR spectroscopy. Finally, 6Li stimulated echo NMR experiments were carried out to record translational two-time correlation functions for the first time. These together with 7Li spin-alignment echo NMR and first 7Li 1D spin-alignment exchange NMR measurements were used to study the interlayer process in Li3N single crystals. (orig.)