DFT-based Metadynamics simulation of proton diffusion in tetragonal zirconia at 1500Â K

The diffusion rate of hydrogen in zirconium oxides comes into play in both the steam oxidation and the hydriding of zirconium alloys. In view of the low measured uptake and diffusion of neutral hydrogen species in zirconium oxides, it has been suggested that the measured rates of hydrogen uptake in zircaloys exposed to high-temperature steam can be explained by the diffusion of protons through the surface oxide layers. This paper investigates the diffusion of protons in tetragonal zirconia at 1500 K using density functional theory-based molecular dynamics and Metadynamics simulations. An average value of 4 × 10−9/m2 s−1 is calculated for the diffusion rate, which, considering the contrived basis of the simulations, compares qualitatively with the value of 3.2 × 10−10/m2 s−1 obtained by fit to experimentally-determined diffusivities of hydrogen in yttrium-stabilized zirconia. The simulations described show that the “proton” is present as the hydrogen atom in a hydroxide ion, and the analysis of the electronic structure reveals that the diffusion of the proton is mediated by two-electron-three-centre bonds that form between hydroxide and adjacent oxide ions.