Nanoscale hydride formation at dislocations in palladium:Ab initiotheory and inelastic neutron scattering measurements

Abstract

Hydrogen arranges at dislocations in palladium to form nanoscale hydrides, changing the vibrational spectra. An ab initio hydrogen potential energy model versus Pd neighbor distances allows us to predict the vibrational excitations for H from absolute zero up to room temperature adjacent to a partial dislocation and with strain. Using the equilibrium distribution of hydrogen with temperature, we predict excitation spectra to explain new incoherent inelastic neutron-scattering measurements. At 0K, dislocation cores trap H to form nanometer-sized hydrides, while increased temperature dissolves the hydrides and disperses H throughout bulk Pd.

Direct Measurement of Hydrogen Dislocation Pipe Diffusion in Deformed Polycrystalline Pd Using Quasielastic Neutron Scattering

“…Extrapolation of the Ann-1/Ann-2 Arrhenius response to higher temperature demonstrates the 350-and 400-K measurements of the Def-1 sample are dominated by bulk hydrogen diffusion, not hydrogen DPD. Hydrogen de-trapping above T ∼ 300 K leads to bulk diffusion behavior in deformed Pd, consistent with the elasticity model for hydrogen occupation of dislocation traps in Pd [20]. We therefore include the two highest T measurements of Def-1 in the fit of the bulk hydrogen diffusion Arrhenius.…”

“…The time-of-flight neutron backscattering silicon spectrometer (BASIS) at the Spallation Neutron Source at Oak Ridge National Laboratory [18] and the High-Flux Backscattering Spectrometer (HFBS) at the National Institute of Standards and Technology Center for Neutron Research [19] were used to quantify trapped hydrogen kinetics with QENS. The sample material for the BASIS and HFBS experiments was 0.25-mm-thick 99.98% pure (metals basis) cold-rolled polycrystalline Pd sheet supplied by Alpha Aesar identical to that used previously for vibrational density of state measurements of trapped hydrogen [20][21][22]. In addition to the deformation by cold rolling, the deformed samples were cycled across the Pd-H miscibility gap (hydride cycling) to introduce additional dislocations using the procedure described in Ref.…”

“…It is difficult to reconcile this observation with site blocking on the octahedral interstitial sublattice in bulk Pd hydride. Site blocking could influence the diffusivity since trapped hydrogen is in the hydride phase below approximately 250 K in PdH 0.001 [20][21][22][23]30]. The diffusion constant D 0 includes the probability of neighboring site availability, 1 − x.…”

“…This behavior is due to a E a ðC H Þ dependence since the D 0 values for both deformed samples are equal to within experimental uncertainty. A continuum of lattice sites associated with dislocations under tensile strain ε exist, from ε ∼ 0 far from dislocations, through ε ∼ r −1 at near-core sites (the inverse radial dependence of volumetric strain associated with edge dislocations [32]), to ε ¼ 0.05 at the dislocation core [20,33]. The occupation of these sites by hydrogen is both T and C H dependent and this occupation dictates the observed E a for DPD between 150 and 300 K. Hydrogen has lower occupation in the core and near-core sites at lower C H .…”

Ab initiomodeling of quasielastic neutron scattering of hydrogen pipe diffusion in palladium

“…6 However, dislocations do not always lead to faster diffusion; one computational study used an embedded atom model potential and found reduced hydrogen diffusion along both screw and edge dislocations in Fe due to high diffusion barriers. 7 Previous studies of hydrogen near a palladium dislocation found the formation of hydrides in the dislocation core, 8 which could block pipe diffusion. Heuser et al…”