STRUCTURE AND LOCAL IONIC MOTION IN MOLYBDENUM CLUSTER CHALCOGENIDES LI1MO6X8 STUDIED BY LI-7 NMR

Solid state Li-7 NMR spectra and relaxation times have been measured for the ternary molybdenum cluster chalcogenides Li1Mo6S8 and Li1Mo6Se8 at 117 and 16 MHz and various temperatures. The powder patterns are governed by chemical shift, quadrupole and dipole interactions, and the asymmetric EFG (or quadrupole interaction) tensor changes toward axial symmetry if going from 100 K to 300 K. Spectra simulations have lead to the conclusion that the intercalated lithium ions reside in one of the six equivalent inner tetrahedral sites within the cavicities around the unit cell origin. The correlation times of local jump motions between these positions follow an Arrhenius plot over several orders of magnitude with activation of 11 kJ/mol (Li1Mo6S8) and 7 kJ/mol(Li1Mo6Se8).