The atomic simulation environment (ASE) is a software package written in the Python programming language with the aim of setting up, steering, and analyzing atomistic simulations. In ASE, tasks are fully scripted in Python. The powerful syntax of Python combined with the NumPy array library make it possible to perform very complex simulation tasks. For example, a sequence of calculations may be performed with the use of a simple 'for-loop' construction. Calculations of energy, forces, stresses and other quantities are performed through interfaces to many external electronic structure codes or force fields using a uniform interface. On top of this calculator interface, ASE provides modules for performing many standard simulation tasks such as structure optimization, molecular dynamics, handling of constraints and performing nudged elastic band calculations.

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 ...

Hydrogen dynamics in crystalline calcium borohydride can be initiated by long-range diffusion or localized motion such as rotations, librations, and vibrations. Herein, the rotational and translational diffusion were studied by quasielastic neutron scattering (QENS) by using two instruments with different time scales in combination with density functional theory (DFT) calculations. Two thermally activated reorientational motions were observed, around the 2-fold (C 2 ) and 3-fold (C 3 ) axes of the BH 4 -units, at temperature from 95 to 280K. The experimental energy barriers (Ea C2 ) 0.14 eV and Ea C3 ) 0.10 eV) and mean residence times are comparable with those obtained from DFT calculations. Long-range diffusion events, with an energy barrier of E aD ) 0.12 eV and an effective jump length of ∼2.5 Å were observed at 224 and 260 K. Three vacancymediated diffusion events, H jumps between two neighboring BH 4 -, and diffusion of BH 4 -and BH 3 groups were calculated and finally discarded because of their very high formation energies and diffusion barriers. Three interstitial diffusion processes (H, H 2 , and H 2 O) were also calculated. The H interstitial was found to be highly unstable, whereas the H 2 interstitial has a low energy of formation (0.40 eV) and diffusion barrier (0.09 eV) with a jump length (2.1 Å) that corresponds well with the experimental values. H 2 O interstitial has an energy of formation of -0.05 eV, and two different diffusion pathways were found. The first gives a H jump distance of 2.45 Å with a diffusion barrier of 0.68 eV, the second one, more favorable, exhibits a H jump distance of 1.08 Å with a barrier of 0.40 eV. The correlation between the QENS and DFT calculations indicates that, most probably, it is the diffusion of interstitial H 2 that was observed. The origin of the interstitial H 2 might come from the synthesis of the compound or a side reaction with trapped synthesis residue leading to the partial oxidation of the compound and hydrogen release.