Metal borohydrides are of interest as hydrogen storage materials due to their high volumetric and gravimetric capacity. However, as with many of the complex hydrides, they are hampered by slow absorption and desorption kinetics and poor reversibility. 1À3 Among the borohydrides, Mg(BH 4 ) 2 ( Figure 1) and Ca(BH 4 ) 2 have more favorable thermodynamic stability than, for example, LiBH 4 , while maintaining attractive hydrogen storage capacities (14.9 and 11.5 mass %, respectively). 2,4 Furthermore, for these two compounds, partial reversibility has been obtained by utilizing high pressure 5À7 (60% 7 À70% 6 recovery of the borohydride) and in the case of Ca(BH 4 ) 2 at more moderate conditions by addition of catalysts (60% recovery 8 ). Kinetic properties have also been shown to improve by using composite materials like Ca(BH 4 ) 2 + MgH 2 . 9 Borohydrides are largely ionic compounds with a general formula M(BH 4 ) n , consisting of metal cations M n+ , the hydrogen atoms being covalently bound to the boron, forming tetrahedral BH 4 À . The possible hydrogen dynamics are long-range translational diffusion and localized motions such as rotations of the BH 4 À complexes along specific axes, librations of the complexes, and vibrations within the complexes. Rotational dynamics are often coupled to orderÀdisorder phase transition in coordination compounds, 10,11 and the decomposition of borohydrides could possibly involve long-range diffusion of H and/or of the whole BH 4 À complex. The first results published on the rotational reorientation of the BH 4 À unit in borohydrides date back to the 1950s. At that time, researchers were interested in understanding the nature of the interactions influencing molecular reorientations in solids. To our knowledge, the first experimental study ever published, giving the energy barriers for the reorientations of BH 4 À in sodium, potassium, and rubidium borohydrides, was performed using nuclear magnetic resonance (NMR) and date from 1955, 12 while a second was published on lithium, sodium, and potassium borohydrides 14 years after, ABSTRACT: In this work, hindered rotations of the BH 4 À tetrahedra in Mg(BH 4 ) 2 were studied by quasielastic neutron scattering, using two instruments with different energy resolution, in combination with density functional theory (DFT) calculations. Two thermally activated reorientations of the BH 4 À units, around the 2-fold (C 2 ) and 3-fold (C 3 ) axes were observed at temperatures from 120 to 440 K. The experimentally obtained activation energies (E aC 2 = 39 and 76 meV and E aC 3 = 214 meV) and mean residence times between reorientational jumps are comparable with the energy barriers obtained from DFT calculations. A linear dependency of the energy barriers for rotations around the C 2 axis parallel to the MgÀMg axis with the distance between these two axes was revealed by the DFT calculations. At the lowest temperature (120 K) only 15% of the BH 4 À units undergo rotational motion and from comparison with DFT results it is expectedly the BH 4 ...
