Atomic Motions in LiBH4 by NMR
1H, 7Li, and 11B 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 7Li spectra, spin−echo T2, and T1 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 °C, a rapidly decreasing T1D (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 °C; the hydrogen diffusion is likely relevant to the kinetics of dehydriding. The extent of 11B line-narrowing demonstrates that the boron atoms also diffuse rapidly at temperatures above 200 °C. In the LT phase, the hydrogen T1D decreases rapidly with increasing temperature, here due to 7Li diffusion which is too slow for line-narrowing.
© 2008 American Chemical Society. Received: September 5, 2008; Revised Manuscript Received: October 6, 2008. Publication Date (Web): November 4, 2008. The authors gratefully acknowledge funding under Grant No. DE-FG02-05ER46256 from the U.S. Department of Energy, Basic Energy Science.