Ab initio study of hydrogen and helium diffusion in Be$_2$Ti

Interstitial hydrogen and helium diffusion in the Be$_2$Ti compound was investigated via ab initio methods. Under certain conditions, this phase can coexist within the desired Be$_{12}$Ti compound, which is a candidate neutron multiplier material for breeder blankets in the DEMO reactor. The Be$_2$Ti lattice contains three stable interstitial hydrogen sites and one stable interstitial helium site, all exhibiting lower solution energies than those found in pure beryllium. This indicates a higher solubility of both hydrogen and helium in Be$_2$Ti. Diffusion barriers between adjacent hydrogen/helium interstitial sites are calculated using a dimer method. At low concentrations, interstitial hydrogen predominantly diffuses through the energetically favorable interstitial sites A, forming a connected network, with an inter-hexagonal barrier of 0.19 eV. At higher concentrations and elevated temperatures, the diffusion involves less energetically favorable interstitial sites B and C, with higher energy barriers of 0.39 and 0.44 eV, respectively. Interstitial helium diffusion is controlled solely by inter-hexagonal jumps with a barrier of 0.52 eV, while the intra-hexagonal barrier is negligible. ... mehrThe energy barriers between adjacent non-equivalent interstitial hydrogen sites A are at least two times lower than the rate-limiting energy barrier in pure beryllium (0.42 eV), suggesting a higher diffusion rate in Be$_2$Ti.