Fast Li diffusion in crystalline LiBH4 due to reduced dimensionality: Frequency-dependent NMR spectroscopy

The hexagonal and orthorhombic form of lithium borohydride, LiBH4, are investigated by temperature and frequency-dependent nuclear magnetic resonance (NMR) spectroscopy. The local electronic structure and microscopic diffusion parameters are determined by recording both Li-6,Li-7 NMR spectra and spin-lattice relaxation (SLR) rates. The rates of the high-temperature flank of the SLR-NMR peaks of hexagonal LiBH4 clearly depend on resonance frequency which unequivocally reveals a low-dimensional diffusion process. Due to the very limited number of suitable model substances this makes lithium borohydride an extremely attractive material to study the effect of reduced dimensionality on Li dynamics. Most likely, the spatial confinement of Li hopping is also responsible for the very high ionic conductivity found for the hexagonal polymorph, recently.