Intergranular structure of nanocrystalline layered Li xTiS 2 as derived from 7Li NMR spectroscopy

Abstract

7Li NMR spectra of ball-milled nanocrystalline Li x TiS 2 have been recorded in the temperature range 140⩽ T⩽500 K. Above 250 K, the central transition of the spectra decomposes into a broad and a narrow component at the same centre frequency. The relative intensity of the narrow line increases with temperature, finally reaching a saturation value of 50%. The narrow line is attributed to more mobile spins in the interfacial regions between the nanocrystalline grains. The temperature dependent intensity of the narrow component is interpreted in terms of an inhomogeneous interface structure. The average particle size of nanocrystalline Li x TiS 2 as obtained from X-ray diffraction (XRD) is 11.8 nm. A simple geometrical model involving the particle diameter d (from XRD), the interface fraction f (from NMR), and the interface thickness g predicts unreasonably thick interfaces but is in accordance with the measured data for d and f when it is adapted to account for a disk-like particle shape. On the basis of the disk model, which is likely to apply for a layered structure like Li x TiS 2, the interface thickness is limited by 1⩽ g⩽3 nm. The results are compared to those obtained for some three-dimensional structures, to which the standard spherical model is applicable in most cases.