NMR Study of Reorientational Motion in Alkaline-Earth Borohydrides: β and γ Phases of Mg(BH₄)₂ and α and β Phases of Ca(BH₄)₂

To study the reorientational motion of BH 4 groups in β and γ phases of Mg(BH 4 ) 2 and in α and β phases of Ca(BH 4 ) 2, we have performed nuclear magnetic resonance (NMR) measurements of the 1 H and 11 B spin–lattice relaxation rates in these compounds over wide ranges of temperature and resonance frequency. It is found that at low temperatures the reorientational motion in β phases of Mg(BH 4 ) 2 and Ca(BH 4 ) 2 is considerably faster than in other studied phases of these alkaline-earth borohydrides. The behavior of the measured spin–lattice relaxation rates in both β phases can be satisfactorily described in terms of a Gaussian distribution of activation energies E a with the average E a values of 138 meV for β-Mg(BH 4 ) 2 and 116 meV for β-Ca(BH 4 ) 2 . The α phase of Ca(BH 4 ) 2 is characterized by the activation energy of 286 ± 7 meV. For the novel porous γ phase of Mg(BH 4 ) 2, the main reorientational process responsible for the observed spin–lattice relaxation rate maximum can be described by the activation energy of 276 ± 5 meV. The barriers for reorientational motion in different phases of alkaline-earth borohydrides are discussed on the basis of changes in the local environment of BH 4 groups.