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

Abstract

Understanding the underlying mechanism of the nanostructure-mediated high diffusivity of H in Pd is of recent scientific interest and also crucial for industrial applications. Here, we present a decisive scenario explaining the emergence of the fast lattice-diffusion mode of interstitial H in face-centered cubic Pd, based on the quantum mechanical natures of both electrons and nuclei under finite strains. Ab initio path-integral molecular dynamics was applied to predict the temperature-and strain-dependent free energy profiles for H migration in Pd over a temperature range of 150-600 K and under hydrostatic tensile strains of 0.0%-2.4%; such strain conditions are likely to occur in real systems, especially around the elastic fields induced by nanostructured defects. The simulated results revealed that, for preferential H location at octahedral sites, as in unstrained Pd, the activation barrier for H migration (Q) was drastically increased with decreasing temperature owing to nuclear quantum effects. In contrast, as tetrahedral sites increased in stability with lattice expansion, nuclear quantum effects became less prominent and ceased impeding H migration. This implies that the nature of the diffusion mechanism gradually changes from quantum-to classical-like as the strain is increased. For H atoms in Pd at the hydrostatic strain of ∼2.4%, we determined that the mechanism promoted fast lattice diffusion (Q = 0.11 eV) of approximately 20 times the rate of conventional H diffusion (Q = 0.23 eV) in unstrained Pd at a room temperature of 300 K.

Suppression of hysteresis in absorption of hydrogen by a Pd-Au alloy

“…In agreement with experiment [9], the theory indicates that the octahedral (O) absorption sites are energetically more favorable than the tetrahedral (T) sites at H/Pd ratios up to 1 [10][11][12][13]. In addition, the theory was used to clarify the specifics of the band structure [12] and phonon dispersion of PdH [12,14,15], the dynamics of H diffusion [16], the effect of lattice strain on H diffusion [11], the energetics of subsurface H absorption [17,18], and the effect of vacancies on H absorption [14,19]. (ii) For pure Au, DFT is in favor of H absorption on the T sites, whereas the H location in the O sites is predicted to be unstable [13,19].…”

Structure of a single palladium nanoparticle and its dynamics during the hydride phase transformation

“…In fact, the presence of strain can affect the hydrogen diffusivity. For example, Kimizuka et al 35 , with an ab initio molecular dynamics model, found that for H atoms in Pd, a hydrostatic strain of 2.4% enhanced the hydrogen lattice diffusion rate by around 20 times that of unstrained Pd at RT.…”

“…In the β phase of palladium hydride, the H atoms occupy two possible interstitial sites in the fcc lattice structure of Pd: octahedral (O) and tetrahedral (T). The diffusion process at the atomic level occurs by thermally activated jumps of H atoms between T and O sites, being the O-sites more thermodynamically stable [35][36][37] and therefore, for the bulk β phase, only the O sites are occupied by the H atoms under ambient conditions 37,38 . The difference between the energy for hydrogen in O and T sites is substantially decreased as strain in increased in the Pd crystalline structure 35 .…”

Analysis for Reverse Temperature Dependence of Hydrogen Permeability through Pd-X (X = Y, Ho, Ni) Alloy Membranes Based on Hydrogen Chemical Potential

“…The large atomic size of Y in Pd expands the face-centered cubic (fcc) crystal lattice. Recently, Kimizuka et al reported that the expansion of Pd lattice stabilizes the tetrahedral sites and enhance the hydrogen diffusivity [30]. The lattice expansion by the addition of Y might provide such an effect of the hydrogen diffusivity.…”