A neutron scattering study of hydrogen dynamics in coarse-grained and nanostructured ZrCr₂H₃

Abstract

The vibrational spectra of hydrogen and parameters of H diffusion in the coarse-grained C15-type system ZrCr(2)H(3) and in nanostructured ZrCr(2)H(3) have been studied by means of inelastic and quasielastic neutron scattering. It is found that the diffusive motion of hydrogen in coarse-grained ZrCr(2)H(3) can be described in terms of at least two jump processes: a fast localized H motion with the jump rate τ(l)( - 1) over the hexagons formed by interstitial Zr(2)Cr(2) sites and a slower process with the rate τ(d)( - 1) associated with H jumps leading to long-range diffusion. While τ(d)( - 1)(T) in the range 250-380 K follows the Arrhenius law with the activation energy of 142 ± 4 meV, the temperature dependence of τ(l)( - 1) deviates from Arrhenius behavior. The nanostructured ZrCr(2)H(3) samples prepared by ball milling consist of C15-type grains and strongly distorted (amorphous-like) regions. H atoms in the strongly distorted regions are found to be immobile on the time scale of our experiments. The microscopic picture of H jump motion in the C15-type grains of the nanostructured samples is similar to that in coarse-grained ZrCr(2)H(3); however, the ball milling leads to a considerable decrease in the jump rate τ(d)( - 1).

Quasi-Elastic Neutron Scattering Studies of Hydrogen Dynamics for Nanoconfined NaAlH4

“…Alternatively, it could describe the very rapid diffusional motion of hydrogen in nanoconfined NaAlH 4 systems. Attempts to fit these data to the Chudley-Elliot , model and the Singwi-Sjolander (S–S) ,, model were not successful and yielded poor fits. A modified S–S model described recently by Shi et al and originally by Hempelmann was also fitted to the data (shown in Figure ) and yielded unreasonably short jump lengths ( l ) of 0.58 Å (at 350 K) to 0.62 Å (at 400 K).…”

Hydrogen dynamics in Ce2Fe17H5: inelastic and quasielastic neutron scattering studies

“…This result suggests that the immobilization of some H atoms at the 18g sites occurs due to defects in the host-metal lattice rather than H-H interactions. Note that an increase in the fraction of immobile H atoms with increasing level of lattice distortions has been reported for the ball-milled Laves-phase system ZrCr 2 H 3 [27]. Host-lattice distortions are expected to distort the g-site hexagons, making some of them unsuitable for the fast localized H motion.…”

“…The increase in p with increasing T was also observed for localized H motion in Laves-phase hydrides [12,13,15,24], α-ScH x [26], and α-LaNi 5 H x [25]; in these cases, the presence of the immobile fraction 1 − p was attributed to H-H interactions leading to the formation of some ordered H configurations, which are destroyed at higher temperatures due to thermal fluctuations. For Laves-phase hydrides, this interpretation is supported by the observed decrease in the value of p with increasing H concentration [12,27]. However, such an interpretation cannot be extended to the case of R 2 Fe 17 H 5 , where each g-site hexagon is occupied by two H atoms.…”