Study of the different types of hydrogen motion in the β-LaNi 5 and β-LaNi 4.5Al 0.5 hydrides by quasi-elastic neutron scattering

Abstract

Quasi-elastic neutron scattering measurements were performed on the β-LaNi 5 and β-LaNi 4.5Al 0.5 hydrides using two different energy resolutions. We show that high resolution spectra are composed of at least three components: one elastic line and two lorentzian components. The broader component is attributed to a rapid local motion, and it is studied using a lower resolution as a function of temperature for the aluminium-substituted hydride. Three jump models are discussed for this motion. The dynamic behaviour of hydrogen in both hydrides is well described in the framework of a structural model in which the five available interstices are grouped into two sets of sites, one concentrated around the z = 1 2 plane and the other consisting of clusters of 3f and 12n sites near the basal plane. We show that there are two types of hydrogen atoms: one type exhibits a diffusive motion on the sites of the first set, and the other performs rapid localized jumps within the 3f–12n site clusters and, within the time scale of our experiment, does not appear to diffuse from the clusters towards the z = 1 2 plane. The total scattering law corresponding to this description is given and is compared with our high resolution data. The activation energy for the rapid local motion in the β-LaNi 4.5Al 0.5 hydride is 170 ± 5 meV.