Li jump process in h- Li0.7 Ti S2 studied by two-time Li7 spin-alignment echo NMR and comparison with results on two-dimensional diffusion from nuclear magnetic relaxation

7Li spin-alignment NMR is used to trace ultraslow diffusion of Li+ in the layered Li conductor LixTiS2 (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 10−1to103s−1 between temperatures T=148 K and 213K. 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)×1012s−1. Altogether, a solitary Li diffusion process was found between 148 and 510K. Li diffusion was investigated in a dynamic range of about 10 orders of magnitude, i.e., 0.1⩽τ−1⩽7.8×108s−1. Additionally, the analysis of final-state echo amplitudes of the two-time correlation functions revealed information about the Li diffusion pathway in Li0.7TiS2. 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. © 2008 The American Physical Society

Atomic-scale measurement of ultraslow Li motions in glassy LiAlSi2 O6 by two-time L6 i spin-alignment echo NMR correlation spectroscopy

6Li spin-alignment echo (SAE) nuclear-magnetic-resonance (NMR) spectroscopy is used to monitor single-particle two-time correlation functions in LiAlSi2O6 glass. The method, here applied in the temperature range from 300 to 400 K, is sensitive to ultraslow Li hopping processes with rates (1/τSAE) down to 10 jumps/s. The use of a sample with natural 6Li abundance allowed the measurement of pure NMR spin-alignment echoes which are damped with increasing mixing time exclusively by slow Li jumps, i.e., free of influences arising from, e.g., interfering spin-diffusion effects. The considerably stretched correlation functions reveal the presence of a broad distribution of jump rates. The results are comprehensively compared with those recently obtained from both 7Li SAE and 7Li spin-lattice relaxation NMR as well as from dc conductivity measurements. Interestingly, the activation energy of the latter, which are sensitive to long-range Li transport parameters, is in good agreement with that microscopically probed by 6Li SAE NMR, here. © 2008 The American Physical Society

Electric field gradient calculations for LixTiS2 and comparison with 7Li NMR results

The elements of the electric field gradient tensor at Li position in the intercalation compound LixTiS2 (with x=0.25, 0.33, 0.67, and 1.0) were calculated with first-principles methods and periodic supercell models. The theoretical results obtained with density functional and Hartree-Fock hybrid methods were compared with experimental field gradients extracted from 7Li NMR spectra from the literature and from our measurements presented here. The dependence of calculated field gradients on the basis set and the explicit form of the exchange-correlation density functional was investigated. In agreement with earlier studies a pronounced effect of polarization functions at the Li site was observed. After optimization of internal degrees of freedom in LiTiS2 all methods under consideration give quadrupole coupling constants in close agreement with experiment. For x < 1 the calculated quadrupole coupling constants were found to depend more sensitively on the method which was attributed to differences in the description of spin localization. The calculations allow one to distinguish between Li atoms placed at octahedral and tetrahedral interstitial sites of the host lattice TiS2. © 2004 American Physical Societ

From ultraslow to fast lithium diffusion in the 2D ion conductor Li0.7TiS2 probed directly by stimulated-echo NMR and nuclear magnetic relaxation

7Li stimulated-echo NMR and classical relaxation NMR techniques are jointly used for the first time for a comprehensive investigation of Li diffusion in layer-structured Li0.7TiS2. One single 2D Li diffusion process was probed over a dynamic range of almost 10 orders of magnitude. So far, this is the largest dynamic range being measured by 7Li NMR spectroscopy directly, i.e., without the help of a specific theoretical model. The jump rates obey a strict Arrhenius law, determined by an activation energy of 0.41(1)  eV and a preexponential factor of 6.3(1)×1012  s−1, and range between 1×10−1  s−1 and 7.8×108  s−1 (148–510 K). Ultraslow Li jumps in the kHz to sub-Hz range were measured directly by recording 7Li spin-alignment correlation functions. The temperature and, in particular, the frequency dependence of the relaxation rates fully agree with results expected for 2D diffusion. © 2006 The American Physical Society

Solid-state diffusion and NMR

Diffusion in solids, which requires the presence of crystal defects or disorder, has both microscopic and macroscopic aspects. Nuclear magnetic resonance techniques provide access to microscopic diffusion parameters like atomic jump rates and activation energies as well as to the tracer diffusion coefficient for macroscopic transport. Microscopic NMR methods include spin-lattice relaxation spectroscopy of stable and beta-radioactive nuclei, spin-spin relaxation or linewidth and spin alignment decay measurements, whereas macroscopic NMR methods are represented by the techniques of static and pulsed field gradient NMR. We recall some basic principles of the mentioned techniques and review case studies for their application to various solids like glassy and crystalline aluminosilicates, nanocrystalline composites, an intercalation compound and a simple bcc metal. Taken together, jump rates in solids are covered over about 10 decades by the microscopic, and diffusion coefficients over 3 decades by the macroscopic NMR methods