Hydrogen-Enhanced Vacancy Diffusion in Metals

• Published in: The journal of physical chemistry letters Vol. 11; no. 17; pp. 7015 - 7020
• Main Authors: Du, Jun-Ping, Geng, W. T, Arakawa, Kazuto, Li, Ju, Ogata, Shigenobu
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 03.09.2020
• Subjects: Defects in solids; Diffusion; Genetics; Hydrogen; INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; Metals; Physical Insights into Materials and Molecular Properties
• ISSN: 1948-7185; 1948-7185
• DOI: 10.1021/acs.jpclett.0c01798

Vacancy diffusion is fundamental to materials science. Hydrogen atoms bind strongly to vacancies and are often believed to retard vacancy diffusion. Here, we use a potential-of-mean-force method to study the diffusion of vacancies in Cu and Pd. We find H atoms, instead of dragging, enhance the diffusivity of vacancies due to a positive hydrogen Gibbs excess at the saddle-point: that is, the migration saddle attracts more H than the vacancy ground state, characterized by an activation excess ΓH m ≈ 1 H, together with also-positive migration activation volume Ωm and activation entropy S m. Thus, according to the Gibbs adsorption isotherm generalized to the activation path, a higher μH significantly lowers the migration free-energy barrier. This is verified by ab initio grand canonical Monte Carlo simulations and direct molecular dynamics simulations. This trend is believed to be generic for migrating dislocations, grain boundaries, and so on that also have a higher capacity for attracting H atoms due to a positive activation volume at the migration saddles.