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

Abstract

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.