Reduction of the repulsive interaction as origin of helium trapping inside a monovacancy in BCC metals

Ab initio simulations of the structure, energetics and mobility of radiation-induced point defects in bcc Nb

“…4A)) in a similar way as observed before in Cu [47]. The energy difference with respect to other less stable structures is very low (0.03 eV), meaning that those atoms can easily move inside the vacancy.…”

“…Other structures, as one appearing as a double 〈111〉 dumbbell, or cross, with the vacancy at its center, or another showing a double 〈100〉 dumbbell, have been checked, but are less stable or evolve to an equivalent tetrahedral configuration. The great stability of this tetrahedral configuration can be justified by the compact He structure obtained, which leads to a smaller increase of the deformation in the surrounding metallic nearest-neighbour atoms, as explained in a previous paper [47].…”

“…The goal was to obtain the maximum He number that could be stably lodged within until the repulsion among the He atoms is so high that several of them are expelled from the vacancy when the equilibrium structure is reached. Our results showed that nine is that critical number [47].…”

“…However, this possible arrangement is less stable than the configuration with the He in between the two vacancies that build up the divacancy. A possible explanation can be found in a reduction of the He-metal repulsion as it was described before in a monovacancy [47]. Interestingly, the most stable divacancy with a He atom is formed by two vacancies placed in 1NN sites (0.18 eV more stable than the 2NN case).…”

Point defect stability in a semicoherent metallic interface

“…Two considerations about the convergence of the calculations should be remarked. First, the formation energy obtained for the most stable site can be compared with the values 3.14 and 3.94 eV found by the authors for the He atoms in tetrahedral positions in the Nb 21 and Cu single crystals 20 , respectively. The value for Nb (3.14 eV) has been calculated inside an ideal 5x5x5 unit cell using 27 k-points and the best lattice parameter for VASP (3.316Å as mentioned before).…”