Hydrogen diffusion in bulk and nanocrystalline palladium: A quasielastic neutron scattering study

Abstract

The diffusion dynamics of hydrogen in bulk and nanocrystalline palladium has been examined using quasielastic neutron scattering (QENS). With respect to bulk PdH0.73, two relaxation processes were found. For both processes, the variation of the relaxation times with momentum transfer was well reproduced by a model of jump diffusion between adjacent octahedral sites. Upon cooling the fast relaxation fraction decreases. The result suggests that the slow relaxation corresponds to jumps between the ground states and the fast one between excited states. In nanocrystalline PdH0.47 with a size of 8 nm, we found a new fast diffusion process with a smaller activation energy in addition to the one observed in the bulk sample. The new process could be due to the motion of hydrogen atoms in the subsurface region where the potential energy surface is substantially modified by surface strain/distortion effects. This is the first QENS experiment that probes hydrogen diffusion within Pd nanoparticles.

Mechanism of fast lattice diffusion of hydrogen in palladium: Interplay of quantum fluctuations and lattice strain

“…The obtained values arehω 0 = 65 meV for O sites and 146 meV for T sites at h = 0.0%. Notably, the value for the O site at zero strain agrees with previously reported experimental values of 62.4-69 meV [9,61]. This indicates that the anharmonicity of the potential surface around the H-preferred site (i.e., the O site) is effectively incorporated in the present ab initio PIMD calculations by sampling the delocalized states of the H atom.…”

“…Much attention has focused on clarifying the mechanism of the high diffusivity of H in nanostructured Pd, such as nanocrystalline grain boundaries [6][7][8], nanoparticles [9,10], and dislocation cores [11,12]. Kofu et al [9] recently examined the diffusion dynamics of H in 8-nm Pd nanoparticles using quasielastic neutron scattering and observed a fast relaxation process with the small activation energy of 0.12 eV, in addition to a conventional relaxation process (0.24 eV) inside the PdH 0.47 nanoparticles. The face-centered cubic (fcc) lattice (space group: F m3m) of Pd has two possible interstitial sites for H atoms, i.e., octahedral (O) sites (4b positions in Wyckoff notation) and tetrahedral (T) sites (8c positions).…”

“…Although H and deuterium (D) atoms in bulk Pd prefer O sites to T sites, a recent neutron powder diffraction experiment [10] suggested that a relatively large fraction of D atoms were located at T sites in the PdD 0.363 nanoparticles. Because an increase in the lattice constant (by up to 1.9%) was locally observed in the pristine Pd nanoparticle samples [9,10], the lattice expansion/distortion was supposed to affect the ground states and dynamics of H (and D) in some manner. However, the results obtained, so far, do not provide evidence for a consistent interpretation of the primary mechanism for the phenomena involved.…”

Vibrational states of atomic hydrogen in bulk and nanocrystalline palladium studied by neutron spectroscopy

“…The Pd powder used as a bulk sample was obtained from Sigma Aldrich with a specified purity of 99.9 % and the hydrogen gas (purity 99.99 %) from Suzuki Shokan. The Pd nanocrystals used in this work are basically the same as those in the previous heat capacity [54], neutron diffraction [38], and QENS measurements [39]. The nanocrystals were synthesized by the same method descried elsewhere [55].…”

“…Our recent neutron diffraction (ND) work on nanocrystalline PdD x (nano-PdD x ) demonstrated that some of H atoms, probably in the subsurface, are accommodated at the tetrahedral (T ) sites (1/4, 1/4, 1/4) [38]. It was also reported by a quasielastic neutron scattering (QENS) study that an additional fast diffusion process of the H atoms appeared in nano-PdH x [39].…”