Journal Article

PreJuSER-13318

http://join2-wiki.gsi.de/foswiki/pub/Main/Artwork/join2_logo100x88.png Hydrogen Rotational and Translational Diffusion in Calcium Borohydride from Quasielastic Neutron Scattering and DFT Calculations

Blanchard, D. ; Riktor, M. ; Maronsson, J. ; Jacobsen, H. ; Kehres, J. ; Sveinbjörnsson, D. ; Bardaji, E. ; Léon, A. ; Juranyi, F. ; Wuttke, J.FZJ* ; Hauback, B. ; Fichtner, M. ; Vegge, T.

2010
Soc. Washington, DC

This record in other databases:  

Please use a persistent id in citations: doi:10.1021/jp107281v

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, mound the 2-fold (C-2) and 3-fold (C-3) axes of the BH4- 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 similar to 2.5 angstrom 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, H-2, and H2O) 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 angstrom) 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 angstrom with a diffusion barrier of 0.68 eV, the second one, more favorable, exhibits a H jump distance of 1.08 angstrom 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.

Keyword(s): J

Note: This work is based on experiments performed at the Swiss spallation neutron source SINQ, Paul Scherrer Institute, Villigen, Switzerland and at FRMII, JCNS Garching, Germany. The authors would like to acknowledge the European Graduate School for Sustainable Energy Technology and the Nordic Center for Excellence on Hydrogen Storage Materials. The Danish Center for Scientific Computing is acknowledged for super-computer access. The Center for Atomic Materials Design (CAMD) is supported by the Lundbeck Fondation. Financial support by EU-IP NESSHY (Contract #518271) and the ERA-NET project Hy-CO is also gratefully acknowledged.

---

Contributing Institute(s):

1. Streumethoden (IFF-4)
2. JCNS (Jülich Centre for Neutron Science JCNS (JCNS) ; JCNS)

Research Program(s):

1. Großgeräte für die Forschung mit Photonen, Neutronen und Ionen (PNI) (P55)
2. BioSoft: Makromolekulare Systeme und biologische Informationsverarbeitung (P45)

Experiment(s):

1. SPHERES: Backscattering spectrometer (NL6S)

Appears in the scientific report 2010

---