DFT study on the nucleation of He bubbles in Pd: Effect of H and self-interstitial atoms

Highlights: • The nucleation mechanism of He clusters in Pd was studied via DFT calculations. • The self-trapping of He atoms could be observed in Pd with at least five He atoms. • The effects of H atoms and SIAs on the nucleation of He bubbles was researched. • Interstitial H atoms had little influence on the nucleation of He bubbles. • The SIA could promote the aggregation as well as the nucleation of He clusters. The nucleation process of helium (He) clusters in the palladium (Pd) system is investigated via Ab initio calculations in this work. When there are five He atoms around a lattice Pd atom, the self-trapping behavior of He atoms in a pure Pd system can be observed. A Pd-Pd split interstitial along the direction near the He cluster is found when the number of interstitial He atoms reaches six. Interstitial H atoms are considered to have little effect on the nucleation of He clusters due to the negligible binding energy between H and He atoms. Besides, the interstitial H atom near He clusters is knocked out by the displaced Pd atom, which further demonstrates that the influence of low concentration of H atoms on the nucleation of He clusters can be ignored. However, self-interstitial atoms (SIAs) can promote the aggregation of He atoms because of the mutual attraction between SIAs and He atoms. The existence of SIAs increases the local pressure around the cluster so that only four He atoms are needed to emit a lattice Pd atom, thereby promoting the nucleation of He clusters.