Quasielastic neutron scattering study of hydrogen motion in A15-typeNb3AlHx

Abstract

In order to study the mechanism and parameters of hydrogen diffusion in A15-type intermetallic Nb 3 Al, we have performed high-resolution quasielastic neutron scattering measurements in Nb 3 AlH x ͑x = 0.13 and 1.77͒ over the temperature range 10-407 K. The experimental results are consistent with a coexistence of two hydrogen jump processes: the fast H motion along the chains formed by interstitial d ͑Nb 4 ͒ sites and the slower H jumps from one chain to another. It is found that the modified Chudley-Elliott model taking into account the structure of the d-site sublattice gives a qualitative description of the experimental data for Nb 3 AlH 0.13 . The faster jump process in Nb 3 AlH 0.13 corresponding to the long-range H diffusion along the d-site chains is characterized by the activation energy of 194 meV. In Nb 3 AlH 1.77 the d-site chains are more than half filled, and the faster jump process is well described in terms of a model of local H jumps within pairs of nearestneighbor d sites on the chains.

Hydrogen Dynamics in Na3AlH6: A Combined Density Functional Theory and Quasielastic Neutron Scattering Study

“…Our DFT calculations result in a prefactor for local H dynamics in Na 3 AlH 6 of τ 0 -1 = 2.23 × 10 13 s -1 , which is 2 orders of magnitude smaller than prefactors for localized events in the Laves phase materials . With an activation energy of 0.37 eV for hydrogen dynamics in Na 3 AlH 6 being at least twice as large compared to activation energies for localized dynamics in the Laves phase materials, the jump rate at, e.g., 390 K, is at least 5 orders of magnitude smaller for localized events in Na 3 AlH 6 . The broadening is therefore still q dependent for larger q compared to the Laves phase materials.…”

“…It should be noted that for localized, low barrier jumps between interstitial sites in other systems with higher symmetry, e.g., as observed in Laves phase materials, one would expect a q independent broadening at large q for localized events. , However, according to the model of successive oscillatory and diffusive motion by Singwi and Sjölander, a smaller diffusion rate can lead to a q dependent broadening also for larger q .…”

“…It should be noted that for localized, low barrier jumps between interstitial sites in other systems with higher symmetry, e.g., as observed in Laves phase materials, one would expect a q independent broadening at large q for localized events. , However, according to the model of successive oscillatory and diffusive motion by Singwi and Sjölander, a smaller diffusion rate can lead to a q dependent broadening also for larger q . With an activation energy of 0.37 eV for hydrogen dynamics in Na 3 AlH 6 being at least twice as large compared to activation energies for localized dynamics in the Laves phase materials, the jump rate at, e.g., 390 K, is at least 5 orders of magnitude smaller for localized events in Na 3 AlH 6 .…”

Nuclear spin relaxation and diffusion of hydrogen in the A15 compound Nb3AlHx

“…The diffusion of H in the cubic A15-type structure Nb 3 AlH x has been studied by nuclear magnetic resonance (NMR) [1] and by quasielastic neutron scattering (QENS) [2]. Nb 3 Al is an example of intermetallic A 3 B compounds with the A15 structure that can absorb large amounts of H, and the aim of the NMR and QENS studies was to understand the H diffusion mechanism in these compounds.…”

“…It was deduced [1,2] that the H occupy tetrahedral interstitial d sites formed by four Nb atoms, and that there are two coexisting processes of jumps between these sites. The faster jump is to one of the two nearest neighbour d sites and the slower jump is to one of the eight second nearest neighbour d sites.…”