UNUSUAL LITHIUM DYNAMICS AND NMR RELAXATION IN A NOVEL CRYSTALLINE THIOBORATE-SULFIDE

The lithium dynamics in the novel thioborate-sulphide double salt Li6+2x[B10S18]S(x) with x almost-equal-to 2 were investigated by temperature and frequency dependent measurements of the Li-7 NMR (and in some cases Li-6 NMR) spectra, and in particular of the relaxation times. By extensive T1, T1rho and T2 studies a large part of the motional spectrum of diffusing Li+ species could be covered. The results show that 20% of the lithium ions have an essentially lower mobility than the others; they could be localized by X-ray diffraction as well and may be considered as part of the crystalline framework. The Li-7 relaxation rates of the 80% highly mobile species reveal similar characteristic deviations from the classical BPP model as those observed in ion conducting glasses. It is concluded that the cations are not localized but randomly distributed among a large number of potential sites within the extended channels of the structure. Many different irregular surroundings and various diffusion paths exist.