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 ¹¹B 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)₂and 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 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 BH₄ groups.

To study the reorientational motion of BH₄ groups in the low-temperature (α) phase of Mg(BH₄)₂, we have performed nuclear magnetic resonance (NMR) measurements of the ¹H and ¹¹B spin−lattice relaxation rates in this compound over wide ranges of temperature (82−443 K) and resonance frequency (14−90 MHz for ¹H and 14−28 MHz for ¹¹B). It is found that the thermally activated reorientational motion in α-Mg(BH₄)₂ is characterized by a coexistence of at least three jump processes with strongly differing activation energies. Taking into account the anisotropy of the local environment of BH₄ groups in α-Mg(BH₄)₂, these jump processes can be attributed to different types of reorientation. The nearly linear coordination of BH₄ groups by two Mg atoms suggests that the fastest jump process corresponds to the rotation around the 2-fold axis connecting B and two Mg atoms, whereas the slowest process is associated with the rotation around two other 2-fold axes perpendicular to the Mg−B−Mg line.