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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: Vortrag
Donnerstag, 3. April 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.