Nuclear Magnetic Resonance Study of Reorientational Motion in α-Mg(BH₄)₂

To study the reorientational motion of BH 4 groups in the low-temperature (α) phase of Mg(BH 4 ) 2, we have performed nuclear magnetic resonance (NMR) measurements of the 1 H and 11 B spin−lattice relaxation rates in this compound over wide ranges of temperature (82−443 K) and resonance frequency (14−90 MHz for 1 H and 14−28 MHz for 11 B). It is found that the thermally activated reorientational motion in α-Mg(BH 4 ) 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 BH 4 groups in α-Mg(BH 4 ) 2, these jump processes can be attributed to different types of reorientation. The nearly linear coordination of BH 4 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.