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

摘要

To study the reorientational motion of BH4 groups in β and γ phases of Mg(BH4)2 and in α and β phases of Ca(BH4)2, we have performed nuclear magnetic resonance (NMR) measurements of the ~1H 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(BH4)2 and Ca(BH4)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(BH4)2 and 116 meV for β-Ca(BH4)2. The α phase of Ca(BH4)2 is characterized by the activation energy of 286 ± 7 meV. For the novel porous γ phase of Mg(BH4)2, the main reorientational process responsible for the observed spin-lattice relaxationrate 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 BH4 groups.