New Lithium Chalcogenidotetrelates, LiChT: Synthesis and Characterization of the Li+-Conducting Tetralithium ortho-Sulfidostannate Li4SnS4

A new lithium chalcogenidotetrelate, denoted as LiChT phase, with the elemental combination Li/Sn/S was synthesized as solvent-free and solvent-containing salts. We present and discuss syntheses, crystal structures, spectroscopic and thermal properties of the phases, as well as the Li+ ion conductivity of Li4SnS4, which is formally related to the thio-LISICON parent system Li4GeS4, and thus represents the first member of a new thiostannate-LISICON family. The solvent-free title compound shows a very promising Li+ ion conductivity of 7 x 10(-5) S.cm(-1) at 20 degrees C and 3 x 10(-3) S.cm(-1) at 100 degrees C, which is exceptionally high for a ternary compound. Activation energies for the lithium ion transport measured via impedance spectroscopy (0.41 eV) correlate reasonably well with the values (0.29 to 0.33 eV) deduced from ionic mobility studies by Li-7 solid-state NMR spectroscopy. NMR two-time correlation functions suggest the occurrence of an additional, geometrically more restricted, ultra-slow-motional process down to 121 K.