Ab initio study of helium in titanium beryllides

Be$_{12}$Ti compound is proposed as a neutron multiplier for tritium-breeding blankets in the demonstration fusion reactor DEMO. Recent experimental studies suggested that Be$_{12}$Ti could contain additions of other phases such as Be$_2$Ti and Be$_{17}$Ti$_2$. In light of these findings, investigation of helium behavior and its binding with vacancy traps in the crystal lattices of these phases is crucial. The paper employs ab initio methods to calculate the helium binding energy with various monovacancy types, as well as the helium solution energies at interstitial sites. The solution energy of helium in all non-equivalent interstitial sites of the titanium beryllides is at least 0.6 eV lower than that for pure beryllium. In the titanium beryllides, helium exhibits stronger binding with the titanium vacancy than with the beryllium vacancy. The binding energy of helium to a vacancy in both Be$_{12}$Ti and Be$_{17}$Ti$_2$ is almost the same as in pure beryllium, except for Be$_2$Ti, which has a lower binding energy. When helium is in the vicinity of a vacancy, it causes the displacement of adjacent beryllium atom into the initial vacancy, while helium substitutes the displaced beryllium atom. ... mehrSome helium atoms may become trapped by a vacancy being outside of it. The obtained results are crucial for the future assessment of interstitial helium diffusion and helium bubble nucleation in titanium beryllides.