Comprehensive NMR study of magnesium borohydride
1H and 11B NMR measurements were performed to study BH4 reorientations and diffusion in four samples of Mg(BH4)2. These include the low (R) and high (β) temperature bulk solid phases, R-phase material with TiF3 and ScCl3 additives, and R-phase material incorporated into a carbon aerogel. All four samples show 1H T1 minima within our temperature range from reorientational motion of the BH4 tetrahedra. The β-phase 1H T1 minimum occurs at a lower temperature than that for the R-phase, indicating a lower activation energy for reorientations. The R-phase 1H spectra display the onset of broadening at low temperatures because of the slowing of reorientations but show no indication of line narrowing at high temperatures from translational diffusion. The β-phase 1H spectra are similar, but here T1D measurements were performed to determine the BH4 hopping rate. Above 150 _C, a significant narrow component does appear in the 1H spectra of the aerogel sample, indicating substantially increased diffusive motions in this component.
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