Grain boundary segregation, stress and stretch: Effects on hydrogen absorption in nanocrystalline palladium

We present a combined experimental and theoretical study of hydrogen solubility at equilibrium in nanocrystalline Pd. We find that the elastic interaction between the grain boundaries and the crystal lattice has important consequences for the adsorption isotherms at small grain size. In α-Pd–H (the dilute solution), the superficial hydrogen density Γ at the grain boundaries is positive, thereby causing tensile stress, as well as an increase in the H fraction in the lattice (when compared with bulk Pd–H). In α′-Pd–H (the concentrated solution), Γ is negative, the stress is compressive and the lattice is depleted of H. We report values for the thermodynamic parameters that quantify the forces and the deformation of the grain boundaries, the interface stress and stretch, respectively. Their values are also of opposite sign for the two phases. Our results for the grain boundary adsorption isotherms imply enrichment or depletion in a layer that is much wider than the structural core of the grain boundary.