Comprehensive NMR Study of Magnesium Borohydride

(1)H and (11)B NMR measurements were performed to study BH(4) reorientations and diffusion in four samples of Mg(BH(4))(2). These include the low (alpha) and high (beta) temperature bulk solid phases, alpha-phase material with TiF(3) and ScCl(3) additives, and alpha-phase material incorporated into a carbon aerogel. All four samples show (1)H T(1) minima within our temperature range from reorientational motion of the BH(4) tetrahedra. The beta-phase (1)H T(1) minimum occurs at a lower temperature than that for the alpha-phase, indicating a lower activation energy for reorientations. The alpha-phase (1)H 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 beta-phase (1)H spectra are similar, but here T(1D) measurements were performed to determine the BH(4) hopping rate. Above 150 degrees C, a significant narrow component does appear in the (1)H spectra of the aerogel sample, indicating substantially increased diffusive motions in this component.