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First-principles calculations have been executed to investigate the interaction between H/He impurities and vacancy in tetragonal Be 12 Ti. The solution energies of four types of most stable H 1 V complexes are lower than that of He 1 V complexes. The binding energies of H atom to four types of vacancies are all lower than that of He atom to vacancies, indicating that He-vacancy complexes can serve as a trap to capture other impurity atoms. Besides, the formation energies of vacancy decrease when a single H or He atom is implanted into vacancy space, and the influence of He atom is greater than that of H atom. The exist of He atom in the H m HeV complexes leads to the increase of solution energies comparing with H m V complexes. In all the H m HeV complexes, the distances between any two H atoms are larger than that in a H 2 molecule, indicating that H 2 molecules can not be formed in these complexes. The trapping behaviours of He-vacancy complexes for H atoms are as follows: a HeV B e 1 and a HeV B e 3 complex can both trap up to three H atoms, a HeV B e 2 complex can capture up to two H atoms, and a HeV Ti complex can capture up to ten H atoms. The number of H atoms trapped by three types of HeV Be complexes reduces compared with that of mono-vacancy for H atoms, indicating that the retention of H atoms within the vacancy space in Be 12 Ti is suppressed by doping of He atom. The present results provide a foundational image of He-vacancy trapping mechanism for H atoms, which contributes significantly to the study on the synergistic effect of hydrogen and helium in tetragonal Be 12 Ti. [ABSTRACT FROM AUTHOR]

• The microstructures of helium bubbles in Be 12 Ti are studied detailedly. • The stabilities of helium bubbles are investigated by the calculations of solution energies. • The nucleation and growth mechanism of helium bubbles in Be 12 Ti are elaborated. First-principles calculations have been conducted to study the stability of He-vacancy clusters and the trapping behavior of vacancies for He atoms in Be 12 Ti. We considered the various metastable microstructures of four types of He-vacancy clusters, and the most stable configurations were determined by the calculations of solution energy. Some He-vacancy clusters in Be 12 Ti have specific geometrical shape, and grow along specific crystal plane. For example, three types of He 2 V clusters (two He atoms trapped by vacancy) form dumbbell-shaped structure, three He atoms of He 3 V clusters (three He atoms trapped by vacancy) are located in the specific crystal plane. The solution energies of four types of clusters increase with the increase of number of He atoms, and the increasing rate is basically identical. The solution energies of He n V B e 1 clusters are always the largest, and the solution energies of the He n V T i clusters are always the smallest, which mainly attribute to the larger vacancy volume and perfect spatial symmetry of Ti vacancies. The binding energies of a single He atom with He n V (n =0-3) clusters are all positive, and the segregation of He atoms on the inner surface of the vacancy is closer to the center of the vacancy, indicating that the He-vacancy has a stronger trapping ability for He atoms. The present research results provide a theoretical foundation of vacancy trapping mechanism for He atoms in Be 12 Ti, which is extremely important for the nucleation and growth mechanism of He bubbles in Be 12 Ti. [ABSTRACT FROM AUTHOR]