Structural and dynamic characterization of Li12Si7 and Li12Ge7 using solid state NMR

Abstract

Local environments and lithium ion dynamics in the binary lithium silicide Li12Si7, and the analogous germanium compound have been characterized by detailed 6Li, 7Li, and 29Si variable temperature static and magic-angle spinning (MAS) NMR experiments. In the MAS-NMR spectra, individual lithium sites are generally well-resolved at temperatures below 200K, whereas at higher temperatures partial site averaging is observed on the kHz timescale. The observed lithium chemical shift ranges of up to 60ppm indicate a significant amount of electronic charge stored on the lithium species, consistent with the expectation of the extended Zintl–Klemm–Bussmann concept used for the theoretical description of lithium silicides. Furthermore the strongly diamagnetic chemical shifts observed for the lithium ions situated directly above the five-membered Si5 rings suggest the possibility of aromatic ring currents in these structural elements. This assignment is confirmed further by 29Si{7Li} CPMAS-heteronuclear correlation experiments. The 29Si MAS-NMR spectra of Li12Si7, aided by 2-D J-resolved spectroscopy, are well suited for differentiating between the individual sites within the silicon framework, while further detailed connectivity information is available on the basis of 2-D INADEQUATE and radio frequency driven recoupling (RFDR) spectra. Variable temperature static 7Li NMR spectra reveal the onset of strong motional narrowing effects, illustrating high lithium ionic mobilities in both of these compounds.