Theoretical investigation on the point defect formation energies in beryllium and comparison with experiments

Aggregation of retained helium and hydrogen in titanium beryllide Be12Ti: a first-principles study

Titanium beryllide, Be₁₂Ti, has been proposed as a prospective neutron multiplier in fusion reactors. First-principles calculations have been performed to investigate the nucleation mechanism of a He bubble in bulk Be₁₂Ti. Meanwhile, the influence of the presence of H atoms on the nucleation of the He bubble, i.e., the synergistic effect of He and H atoms, has also been investigated. It has been found that the He bubble will initially nucleate around a monovacancy (V_Be2). When more He atoms have been implanted, two newly induced vacancies (V_Be1 and V_Be3) could be successively observed. The nucleation of the He bubble will occur around the divacancy of V_Be2V_Be1 and the trivacancy of V_Be2V_Be1V_Be3. Dumbbell structures in the He bubble evolve with the number of implanted He atoms and finally disappear. The presence of H atoms will significantly influence the nucleation of the He bubble. It is interesting that some tetrahedral and octahedral structures have also been observed. The maximal number of H atoms trapped by a He bubble has been obtained. These phenomena could be further explained by the continuous shrinking of the isosurface of charge density. The present results provide a microscopic physical foundation to understand the mechanism of He and H atoms retention in neutron multiplier materials. This investigation could be helpful for the design and fabrication of more promising beryllides which could withstand a severe external environment.

The He bubble will initially nucleate around V_Be2. Then the nucleation of the He bubble will occur around the divacancy of V_Be2V_Be1 and the trivacancy of V_Be2V_Be1V_Be3. The presence of H atoms will significantly influence the nucleation of the He bubble.

Hydrogen in beryllium oxide investigated by DFT: on the relative stability of charged-state atomic versus molecular hydrogen

The behavior of hydrogen in perfect wurtzite beryllium oxide is herein investigated by means of electronic structure calculations based on density functional theory. The formation energies of the following set of states of hydrogen (H⁰, H⁺, H^-, H₂, [Formula: see text], [Formula: see text]) are computed and their solubility is established as a function of temperature and pressure with emphasis given to conditions relevant for hydrogen-implanted materials. It is found that all magnetic states H⁰, [Formula: see text], [Formula: see text] are unstable, while the relative stability of the non-magnetic states depends on the thermodynamic conditions: H₂ prevails above temperatures around 900 K at standard pressure, which is the lowest temperature in experiments measuring the diffusion coefficient of hydrogen in wurtzite beryllium oxide. Under hydrogen implantation, the total concentration of hydrogen is fixed by the implantation source; it is found that molecular hydrogen prevails starting from a very low total concentration in hydrogen (as low as 10^-40 at.fr.). Finally, the diffusion coefficient of H₂ in beryllium oxide is calculated and the results are compared with previous experimental data.