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

TY - JOUR

T1 - Hindered Rotational Energy Barriers of BH4 Tetrahedra in beta-Mg(BH4)(2) from Quasielastic Neutron Scattering and DFT Calculations

AU - Blanchard, D.

AU - Maronsson, J.B.

AU - Riktor, M.D.

AU - Kheres, J.

AU - Sveinbjörnsson, D.

AU - Gil Bardaji, E.

AU - Léon, A.

AU - Juranyi, F.

AU - Wuttke, J.

AU - Lefmann, Kim

AU - Hauback, B.C.

AU - Fichtner, M.

AU - Vegge, T.

PY - 2012/1/19

Y1 - 2012/1/19

N2 - In this work, hindered rotations of the BH 4 - 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 - 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 aC2 = 39 and 76 meV and E aC3 = 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 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 BH 4 - units undergo rotational motion and from comparison with DFT results it is expectedly the BH 4 - 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.

AB - In this work, hindered rotations of the BH 4 - 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 - 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 aC2 = 39 and 76 meV and E aC3 = 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 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 BH 4 - units undergo rotational motion and from comparison with DFT results it is expectedly the BH 4 - 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.

U2 - 10.1021/jp208670v

DO - 10.1021/jp208670v

M3 - Journal article

SN - 1932-7447

VL - 116

SP - 2013

EP - 2023

JO - The Journal of Physical Chemistry Part C

JF - The Journal of Physical Chemistry Part C

IS - 2

ER -