Li jump process in h-Li_(0.7)TiS_2 studied by two-time ~7Li spin-alignment echo NMR and comparison with results on two-dimensional diffusion from nuclear magnetic relaxation

摘要

~7Li spin-alignment NMR is used to trace ultraslow diffusion of Li~+ in the layered Li conductor Li_xTiS_2 (x=0.7). Two-time correlation functions were recorded for fixed evolution times as a function of mixing time at temperatures within the ~7Li rigid-lattice regime. The corresponding decay rates were identified as Li jump rates τ~(-1) ranging from 10~(-1) to 10~3 s~(-1) between temperatures T=148 K and 213 K. The jump rates obtained directly from spin-alignment echo NMR and those from diffusion induced maxima of spin-lattice relaxation peaks, monitored in the laboratory as well as in the rotating frame, are consistent with each other and follow an Arrhenius law with an activation energy of 0.41(1) eV and a preexponential factor of 6.3(1) × 10~(12) s~(-1). Altogether, a solitary Li diffusion process was found between 148 and 510 K. Li diffusion was investigated in a dynamic range of about 10 orders of magnitude, i.e., 0.1 ≤ τ~(-1) ≤ 7.8 × 10~8 s~(-1). Additionally, the analysis of final-state echo amplitudes of the two-time correlation functions revealed information about the Li diffusion pathway in Li_(0.7)TiS_2. Obviously, a two-site jump process is present, i.e., besides the regularly occupied octahedral sites also the vacant tetrahedral ones within the van der Waals gap are involved in the overall two-dimensional diffusion process.