Atomic Motions in LiBH4 by NMR

H-1, Li-7, and B-11 NMR measurements were used to understand atomic translational motions in both the low- and high-temperature phases (LT, HT) of LiBH4. In the HT phase Li-7 spectra, spin-echo T-2, and T-1 all indicate very rapid lithium ion diffusion. Just above the phase transition, the hydrogen resonance is broad, about 22 kHz fwhm (full width at half of maximum), showing that H translations remain slow. From 120 to 170 degrees C, a rapidly decreasing T-1D (relaxation time of dipolar spin-order) shows that the hydrogens diffuse increasingly rapidly. This motion eventually results in marked hydrogen line-narrowing centered near 190 degrees C; the hydrogen diffusion is likely relevant to the kinetics of dehydriding. The extent of B-11 line- narrowing demonstrates that the boron atoms also diffuse rapidly at temperatures above 200 degrees C. In the LT phase, the hydrogen T-1D decreases rapidly with increasing temperature, here due to Li-7 diffusion which is too slow for line-narrowing.