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 ${\mathrm{Nb}}_{3}\mathrm{Al}$, we have performed high-resolution quasielastic neutron scattering measurements in ${\mathrm{Nb}}_{3}{\mathrm{AlH}}_{x}$ ($x=0.13$ and 1.77) over the temperature range $10--407\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The experimental results are consistent with a coexistence of two hydrogen jump processes: the fast H motion along the chains formed by interstitial $d$ $({\mathrm{Nb}}_{4})$ 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 ${\mathrm{Nb}}_{3}{\mathrm{AlH}}_{0.13}$. The faster jump process in ${\mathrm{Nb}}_{3}{\mathrm{AlH}}_{0.13}$ corresponding to the long-range H diffusion along the $d$-site chains is characterized by the activation energy of $194\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$. In ${\mathrm{Nb}}_{3}{\mathrm{AlH}}_{1.77}$ 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 nearest-neighbor $d$ sites on the chains.