The helium-vacancy complexes and helium bubbles formation mechanism in chromium: a comprehensive first-principle study

Synergistic interaction between the helium (He) atom and vacancy (Vac) in chromium (Cr) coatings plays an important role in the evolution of radiation-induced microstructures and He bubbles. First-principle calculations were utilized to investigate the contributions of Vac–Vac He–He and He_n-Vac_m interactions (n = 1 ~ 10, m = 0 ~ 3) to electronic structures and energetics in Cr superlattice. It is found that the vacancy formation energy was dramatically decreased by the He dispersing distribution. The mono-vac (Vac₁) was preferred to be occupied by one He atom, while the vacancy clusters can be formed by the most stable bi-vac (Vac₂) and tri-vac (Vac₃). Two He atoms with the second nearest-neighbor (T ₀–T ₂) structure at equilibrium distances of 1.60 Å was the most stable configuration of the He–He pair. Moreover, when the Vac₁ accommodated 10 He atoms, the improved vacancy volume and lattice distortion may induce to capture more He atoms in the Vac₁, and the He bubble was produced. The present research results provide the physical essence of vacancy aggregation and a theoretical foundation of the vacancy trapping mechanism for He atoms in Cr, which is extremely important for the nucleation and growth mechanism of He bubbles in Cr.