Hydrogen Rotational and Translational Diffusion in Calcium Borohydride from Quasielastic Neutron Scattering and DFT Calculations

Abstract

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 (C2) and 3-fold (C3) axes of the BH4− units, at temperature from 95 to 280K. The experimental energy barriers (EaC2 = 0.14 eV and EaC3 = 0.10 eV) and mean residence times are comparable with those obtained from DFT calculations. Long-range diffusion events, with an energy barrier of EaD = 0.12 eV and an effective jump length of ∼2.5 Å were observed at 224 and 260 K. Three vacancy-mediated diffusion events, H jumps between two neighboring BH4−, and diffusion of BH4− and BH3 groups were calculated and finally discarded because of their very high formation energies and diffusion barriers. Three interstitial diffusion processes (H, H2, and H2O) were also calculated. The H interstitial was found to be highly unstable, whereas the H2 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. H2O 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 H2 that was observed. The origin of the interstitial H2 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.