Retention and diffusion of transmutation H and He atoms in Be 12 Ti: first-principles calculations.

The beryllide Be 12 Ti 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 Be 12 Ti. 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 Be 12 Ti. 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.