Hindered Rotational Energy Barriers of BH4– Tetrahedra in β-Mg(BH4)2 from Quasielastic Neutron Scattering and DFT Calculations

In this work, hindered rotations of the BH4– tetrahedra in Mg(BH4)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 BH4– units, around the 2-fold (C2) and 3-fold (C3) axes were observed at temperatures from 120 to 440 K.

The experimentally obtained activation energies (EaC2 = 39 and 76 meV and EaC3 = 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 C2 axis parallel to the Mg–Mg axis with the distance between these two axes was revealed by the DFT calculations.

At the lowest temperature (120 K) only 15% of the BH4– units undergo rotational motion and from comparison with DFT results it is expectedly the BH4– units with the boron atom closest to the Mg–Mg axis, although dynamics related to local disorder existing at the boundary of the antiphase domains or to the presence of solvent in the sample cannot be strictly excluded.

No long-range diffusion events were observed.