Vacancy-enhanced mechanism for helium diffusion along Σ7 grain boundary in α-Al₂O₃: A first principle study

Helium migration behavior in the bulk and along the rhombohedral Σ7 grain boundary (GB) of α-Al₂O₃ has been studied via first-principles calculations based on the projector-augmented wave method. It turns out that the formation energies of helium in the α-Al₂O₃ bulk and in the rhombohedral Σ7 GB area are 2.12 eV and 3.15 eV, respectively. The energy barrier for helium migration in bulk is 2.28 eV, indicating that helium is difficult to diffuse. Moreover, the calculations also reveal that the radiation-induced O vacancies are favored to aggregate to the Σ7 GB plane, forming a zigzag O vacancy chain. In such circumvent, however, the energy barrier for helium diffusion along vacancy chain is significantly reduced to 0.61 eV, suggesting such O vacancy chain would provide the fast diffusion path for helium to escape from α-Al₂O₃ due to its large positive formation energy in the bulk.