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Dresden 2014 – scientific programme

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MM: Fachverband Metall- und Materialphysik

MM 53: Topical session: X-ray and neutron scattering in materials science II - Atomic migration studies ranging from neutrons to coherent X-rays

MM 53.2: Talk

Thursday, April 3, 2014, 12:15–12:30, BAR 205

Hydrogen motion in LiBH4 across the structural phase transition — •Luca Silvi and Wiebke Lohstroh — Forschungsneutronenquelle Heinz Maier-Leibnitz FRM II, Lichtenbergstrasse 1, 85747 Garching, Germany

Lithium Borohydride (LiBH4) has an intense research activity due to its high hydrogen content (18.4 mass% H2 and 121.3 kg/m3 of gravimetric and volumetric hydrogen density). The slow sorption kinetics and the high temperatures required for hydrogen exchange are major obstacle for everyday-life applications. In LiBH4, hydrogen is bonded covalently to a central Boron atom forming a (negatively charged) tetrahedron, and the BH4 unit bonds ionically to Li ion. By means of Quasielastic Neutron Scattering (QENS), the localized hydrogen motions were investigated as a function of temperature, across the phase structural transition at 380 K. QENS have been performed at the direct time-of-flight neutron spectrometer TOFTOF at FRM-II, and the Elastic Incoherent Structure Factor (EISF) could be determined in a momentum transfer-range up to 4 Å−1. The large dynamical range is essential for the discrimination of different kind of rotational modes. At temperature below the phase transition, 120 rotations occur around the C3 symmetry axis of the BH4 unit. As the phase transition is approached, the dynamic of the BH4 unit changes continuously. At an intermediate state, a tumbling motion of the tetrahedron is observed, while at 413K the hydrogen motion is best described as a diffusion on a surface of a sphere. The results confirm that the structural phase transition is closely linked with the dynamic of the BH4 units.

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