Inverse isotope effect in Pd: First-principles insights into hydrogen isotope diffusion, dissolution, and permeation

Abstract

Utilizing first-principles calculations within the framework of density functional theory, this work systematically investigates the dissolution, diffusion, and permeation of hydrogen (H), deuterium (D), and tritium (T) in palladium. A key finding is the prediction of a distinct inverse isotope effect in diffusion at low temperatures (T < ∼800 K), where the calculated diffusion coefficients increase with the mass of the isotope, contrary to classical expectations. This anomaly is quantitatively traced to the reduction of the effective diffusion barrier with increasing isotope mass due to zero-point energy contributions. In contrast, dissolution and permeation maintain the normal isotope trend. By integrating microscopic calculations with a macroscopic Fick transport model, this study offers crucial insights into the quantum-mechanical origins of isotope effects in metals and guides the design of Pd-based membranes for isotope separation applications.