Hydrogen interactions with intrinsic point defects in hydrogen permeation barrier of α-Al2O3: a first-principles study

It is crucial to understand hydrogen interactions with intrinsic point defects in the hydrogen permeation barrier (HPB) of α-Al2O3, finding underlying reasons for the not-so-low H-permeability of the barrier, and thereby produce samples with tailored defects for optimal performance. Using density functional theory (DFT), the formation energies of intrinsic point defects in an α-Al2O3 lattice, including extrinsic H-related defects (H(i), V(Al)-H complex, HO(i) and H(O)), in all possible charged states, are first calculated under HPB working conditions, to determine the dominant basic defect species for hydrogen. We find that the stable forms of H-related defects in α-Al2O3 are charged H interstitials (H(i)(q), where q is the charge state of the defect) and hydrogenation of the bulk V(Al)(3-) ([V(Al)(3-)-H(+)](q)), under hydrogen-rich conditions. As the system reaches equilibrium, H in α-Al2O3 is mainly present in the H(i)(+) state, and preferentially exists in the form of [V(Al)(3-)-H(+)] and H(O)(+). Migration processes of the dominant defects are further investigated, predicting that H(i)(+) is the predominant diffusion species in α-Al2O3. [V(Al)(3-)-H(+)](2-) and H(O)(+) can release trapped hydrogen during high temperature annealing, contributing to the H-transport in α-Al2O3. The formation energy is much higher than the migration energy for H(i)(+), suggesting that the migration of H(i)(+) is the bottleneck for creating low enough H-permeation in α-Al2O3, and corresponding strategies for optimum H-suppressing performance for an α-Al2O3 HPB are proposed.