Effect of doping Ti on the vacancy trapping mechanism for the helium in ZrCo from first principles

The interactions of dopants with point defects, such as vacancy, helium, et.al, can affect helium evolution, ultimately the macroscopic properties of materials. Here, the microscopic vacancy trapping mechanism for He defects and the formation of small HemVn (consisting of m He atoms and n vacancies) clusters in pure and Ti-doped ZrCo system are investigated by carrying out an extensive set of first-principles calculations based on density functional theory. Our results uncover the following. The helium atom can segregate from the adjacent interstitial (tetrahedral and octahedral) sites towards the vacancy center spontaneously, and therefore a single He atom is energetically favorable to occupy a vacancy whether in pure or doped system. The dopant Ti can act as a trapping center for He impurities as like a vacancy. Moreover, it can improve the trapping ability and increase the trapping radius of the vacancies for helium. As for the effect of the Ti atom on vacancy trapping multiple helium atoms, the higher barrier in doped systems than pure one elucidates that doping inhibits the formation of large HemV clusters. Furthermore, in order to evaluate the effect of dopant Ti on the stability of He atoms in multiple vacancies, the binding energy of a helium atom, a vacancy (V) and a self-interstitial atom (SIA) to a helium-vacancy cluster (HemVn) were obtained and compared with pure system. The results suggest that the cluster growth can be inhibited due to the dopant Ti, and therefore the formation of large helium bubbles is also hindered. All the binding energies do not depend much on cluster size, but primarily on the helium-to-vacancy ratio (m/n) of clusters. The stability of clusters is decided by the competitive processes among the emission of He atoms, vacancies, and SIAs, also depending on the helium-to-vacancy ratio. The present results provide in-depth explanation for the effect of the dopant on helium behavior and could aid future tritium storage material design.