Two-dimensional diffusion in Li sub(0.7)NbS sub(2) as directly probed by frequency-dependent super(7)Li NMR

• Published in: Journal of physics. Condensed matter Vol. 25; no. 19; pp. 1 - 7
• Main Authors: Epp, V, Nakhal, S, Lerch, M, Wilkening, M
• Format: Journal Article
• Language: English
• Published: 15.05.2013
• Subjects: Annealing; Diffusion; Diffusion rate; Dynamics; Lithium; Mathematical models; Nuclear magnetic resonance; Two dimensional
• ISSN: 0953-8984; 1361-648X
• DOI: 10.1088/0953-8984/25/19/195402

Li ion diffusion in layer-structured Li sub(0.7)NbS sub(2) has been complementary investigated by nuclear magnetic resonance (NMR) spectroscopy from an atomic scale point of view. In the present case, super(7)Li NMR spin-lattice relaxation (SLR) rates R sub(1) sub([rho]) probed in the rotating frame of reference proved very informative in characterizing the Li self-diffusion process in the van der Waals gap between the NbS sub(2) layers. While temperature-variable SLR[rho] measurements were used to determine dynamic parameters such as jump rates ([tau] super(-1)) and the activation energy (E sub(a)), frequency-dependent measurements were used to specify the dimensionality of the diffusion process. In particular, the effect of annealing, i.e., the distribution of Li ions between the layers, on overall Li dynamics has been studied. When plotted in an Arrhenius diagram, the R sub(1) [rho] rates of an annealed sample, which were recorded at a locking frequency of 20 kHz, pass through a diffusion-induced relaxation peak whose maximum shows up at 320 K. Employing an appropriate diffusion model and appropriately accounting for a non-diffusive background relaxation, a Li jump rate [tau] super(-1) (300 K) [asymptotically =] 1:3 x 10 super(5) s super(-1) and an activation energy E sub(a) of 0.43(2) eV can be deduced. Most importantly, in the high-T limit of the diffusion-induced rate peak, i.e., when omega sub(1)[tau] << 1 holds, the rates follow a logarithmic frequency dependence. This points to a diffusion process of low dimensionality and is in good agreement with predictions of relaxation models developed for 2D diffusion.