Ab initio lattice-gas modeling of interstitial hydrogen diffusion in CuPd alloys

A comprehensive lattice-gas model has been developed to predict the diffusivities of dilute H in a wide range of structurally disordered fcc CuPd alloys with compositions greater than 47 at. % Pd. We used density functional theory to perform detailed calculations of H binding energies, vibrational frequencies, and activation barriers to local diffusion in two representative alloys having 52 and 74 at. % Pd. These data are used to parametrize all possible hopping rates between adjacent interstitial sites in a lattice model for the entire range of compositions. Kinetic Monte Carlo (KMC) simulations of this lattice model were used to calculate H tracer diffusivities as a function of alloy composition and temperatures ranging from 400 K to 1200 K. The results from these simulations are found to be in good agreement with available experimental data. We have also used KMC simulations to investigate the impact of short-range order on H diffusion in fcc CuPd alloys. The effects of short-range order are found to be small under the conditions of experimental interest.