Retention and diffusion of transmutation H and He atoms in Be₁₂Ti: first-principles calculations

The beryllide Be₁₂Ti is considered to be the most promising candidate material for advanced plasma facing materials in future fusion reactors because of its excellent performance. In this work, first-principles calculations were conducted to gain insight into the retention and diffusion behavior of transmutation H and He atoms in Be₁₂Ti. The solution energy and migration energy of single impurity H/He atoms were computed to study the behavior of their retention and diffusion. Among seven stable interstitial sites, H atoms preferentially occupy the octahedral interstitial site, I _oct, whereas He atoms preferentially occupy the dodecahedral interstitial site, I _dode. The solubility of H is much higher than that of He in Be₁₂Ti. When monovacancy is generated, H atoms preferentially stay in the vicinity of Be1 vacancies, while He atoms tend to reside in the center of Ti vacancies. The migration energy barrier of a single He atom between first near-neighbor I _dode sites is 0.35 eV. For H atoms, the migration energy barrier from I _dode to I _tetra2 is 0.45 eV. The barrier along the paths I _tri1-I _dode-I _tri1 is 0.38 eV. When a Be3 vacancy is introduced as the neighbour of I _tri1, the migration energy barrier increases to 0.77 eV. These results indicate that vacancies can trap impurity atoms and may act as seeds for bubble formation.