Quasielastic neutron scattering study of hydrogen motion in C15-type YMn₂H_x

Abstract

In order to study the mechanism and parameters of hydrogen diffusion in the cubic (C15-type) Laves phase YMn 2 , we have performed quasielastic neutron scattering measurements in YMn 2 H x (x = 0.4, 0.65 and 1.26) over the temperature range 30-395 K. It is found that the diffusive motion of hydrogen in this system can be described in terms of two jump processes: the fast localized H motion with the jump rate τ −1 l and the slower process with the rate τ −1 d associated with H jumps leading to long-range diffusion. The ratio τ d /τ l at room temperature is close to 10 2 . Our results suggest that the localized H motion in YMn 2 H x corresponds to back-and-forth jumps of hydrogen atoms within pairs of interstitial g (Y 2 Mn 2 ) sites. The parameters of the long-range diffusion of hydrogen in the samples with different H content are found to be close to each other. In the range 210-395 K, the temperature dependences of τ −1 d and the hydrogen diffusivity are reasonably described by the Arrhenius law with activation energies 0.18-0.22 eV.

Hydrogen jump motion in Laves-phase hydrides: Two frequency scales

“…The values of r 2 /r 1 are derived from the neutron diffraction data for TaV 2 D x [10], ZrTi 2 D x [24], ZrMo 2 D x [25], [22] and YMn 2 D x [23]. [28] on YMn 2 H x (R A /R B = 1.425, x = 0.4, 0.65 and 1.26) are consistent with these expectations.…”

Observation of Hydride Mobility in the Transition‐Metal Oxide Hydride LaSrCoO3H0.7

“…The mobility of the hydride anion in the lattice is therefore an issue of particular importance in the synthesis of the oxide hydride, as its presence must contribute to the stability of the phase in the highly reducing synthetic environment. Further studies on the mechanism of hydride insertion are warranted by the lack of such information on any known oxide hydride material, including LaHO, [5] Ba 3 (AlO 4 )H, [6] Ba 21 Ge 2 O 5 H 24 , [7] and BaMnO 3 H 3 .[8] Quasielastic neutron scattering (QENS) is a well-established technique for probing diffusion of atoms and molecules in such systems as proton-conducting oxides, [9] metal-hydride systems, [10] ionic polymers, [11] and clay minerals. [12] We report herein that hydride anion mobility within the perovskite layer of LaSrCoO 3 H 0.7 above 675 K has been observed by high-resolution QENS; the observed diffusion allows an estimate of the molar hydride ion conductivity as being on the order of 4 S cm -1 .…”

Introduction to Quasielastic Neutron Scattering

“…Thus we treat the diffusion process as if all diffusing particles start at the origin at time zero; this leads to the correct result because of temporal and spatial invariance. The solution is (21) Spatial Fourier transformation yields the self-part of the intermediate scattering function (22) and the subsequent Fourier transformation with respect to time yields the incoherent scattering function (23) which is a Lorentzian function with the half-width at half maximum (HWHM) (24) At sufficiently small Q this so-called Q 2 -law is generally valid irrespective of the details of the diffusion process, which can be elucidated from measurements at finite or larger Q, but any long-range diffusion model converges to the Q 2 law at small Q.…”

“…From time to time the hydrogen manages to jump to a 3f site in the basal plane and then further to the next hexagon. In a systematic series of experiments Skripov et al [24,25] investigated Laves phase intermetallic hydrides and could show that depending on the lattice constant (atomic radii of the constituent metals either dumbbell-like or hexagon-like localized hydrogen motions occur in the diffusional process.…”