Regensburg 2002 – wissenschaftliches Programm
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M: Metallphysik
M 24: Wasserstoff in Metallen I
M 24.2: Fachvortrag
Donnerstag, 14. März 2002, 09:45–10:00, H4
NMR studies of the hydrogen diffusion in the hydrogen-stabilized Laves phase compound HfTi2Hx (x≈ 4) — •Ulrich Eberle1, Günter Majer1, and A. V. Skripov2 — 1Max-Planck-Institut für Metallforschung, Heisenbergstraße 1, 70569 Stuttgart — 2Institute of Metal Physics, Urals Branch of the Academy of Sciences, Ekaterinburg 620219, Russia
Many cubic Laves-phase compounds C15-AB2 form hydrides with large hydrogen contents. In these systems hydrogen atoms occupy two types of tetrahedral interstitial sites, the g-sites surrounded by two A and two B atoms and the e-sites surrounded by one A and three B atoms. Of special interest is the system C15-HfTi2Hx (x ≈ 4). This system is obtained from the disordered hcp alloy HfTi2 by hydrogen absorption, and the hcp alloy is formed again when the hydrogen is removed. The mechanisms of hydrogen diffusion have been investigated by two methods: a) the measurement of long-range diffusion DT by PFG-NMR (pulsed field gradients) and b) measurements of the spin-lattice relaxation rate Γ1. The Γ1 - data indicate that hydrogen motion occurs on two different time scales. The temperature dependence of the diffusion coefficient DT shows Arrhenius behaviour with Ha ≈ 200 meV. A comparison of the PFG- and Γ1 - data permits to distinguish between long-range diffusion and localized motion. A consistent explanation of the PFG- and Γ1 - data could be obtained under the assumption that besides long-range diffusion via e-sites, localized motion occurs by jumps of hydrogen within a hexagon formed by six g-sites. This model is also able to explain previous QENS-measurements on C15-HfTi2Hx.