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

The interactions of dopants with point defects such as that between vacancies and helium can affect helium evolution and ultimately the macroscopic properties of materials. Herein, the microscopic vacancy trapping mechanism for He defects and the formation of small HemVacn (consisting of m He atoms and n vacancies) clusters in pure and Ti-doped ZrCo systems 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 the pure or in the doped system. The dopant Ti can act as a trapping center for He impurities similar to 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 the trapping of multiple helium atoms by the vacancy, the higher barrier in the doped systems than in the pure one implies that doping inhibits the formation of large HemVac clusters. Furthermore, in order to evaluate the effect of dopant Ti on the stability of He atoms in multiple vacancies, the binding energies of a helium atom, a vacancy (Vac), and a self-interstitial atom (SIA) to a helium-vacancy cluster (HemVacn) were obtained and compared with that of the pure system. The results suggest that the cluster growth can be inhibited by the dopant Ti, and therefore, the formation of large helium bubbles is also hindered. All the binding energies do not depend much on the cluster size but primarily on the helium-to-vacancy ratio (m/n) of the clusters. The stability of the clusters is decided by the competitive processes among the emission of He atoms, vacancies, and SIAs, and also depends on the helium-to-vacancy ratio. The present results provide an in-depth explanation for the effect of the dopant on helium behavior and could aid future tritium storage material design.

Influence of Ti on the dissolution and migration of He in ZrCo based on first principles investigation.

We have performed state-of-the-art ab initio calculations based on density functional theory to study the effect of Ti on helium dissolution and migration in a dilute Ti-doped ZrCo system. The formation energy of He-related defects predicts that it is preferable to occupy the VZr (Zr vacancy) at first. As for the Heint (interstitial site He), the results corroborate that Hetet (tetrahedral site He) is more stable than Heoct (octahedral site He) by 0.25 eV. The Heoct in the vicinity of Ti atoms becomes unstable, being relaxed into a nearby tetrahedral site, unlike in the pure ZrCo. We also reveal that ZrCo is susceptible to dopant Ti in terms of helium diffusion. The energy barrier for a Hetet to diffuse into a neighboring tetrahedral site is found to be about three times as large as the migration barrier between two adjacent octahedral interstitial sites (0.35 vs. 0.12 eV). In addition, the He atom can migrate from one octahedral site to another without going through a tetrahedral one in pure ZrCo. Furthermore, Hetet needs to overcome higher energy barriers of 0.27 eV and 0.58 eV in Ti-doped ZrCo than in the pure one (0.22 eV and 0.35 eV) along the 1nn (the first nearest neighbor) → 1nn → 2nn (the second nearest neighbor) pathway with the He atom escaping away from the Ti region. In addition, the dissociative energy barrier of the HeZr (Zr position substituted by the He atom) or HeCo (Co position substituted by the He atom) is somewhat higher in the presence of Ti than the pure one. All these conclusions elucidate that Ti acts as a trapping center for He impurities and blocks interstitial He mobility in ZrCo alloys.

The performance of adsorption, dissociation and diffusion mechanism of hydrogen on the Ti-doped ZrCo(110) surface.

Compared with pristine ZrCo(110), the adsorption, dissociation, and successive diffusion of hydrogen on the Ti-decorated ZrCo(110) have been investigated based on first-principles calculation. For the purpose of having fast absorption kinetics, both activation processes need to overcome small energy barriers. The adsorption energies of molecular as well as atomic hydrogen on the Ti-decorated ZrCo(110) surface were calculated using first-principles calculations with the periodic density functional theory (DFT). The H2 molecule on ZrCo(110) and Ti-doped ZrCo(110) surfaces could be spontaneously partially dissociated due to the interaction with the substrate surfaces, producing H atoms strongly chemisorbed to the hollow sites. The H2 dissociation energy barrier and the H diffusion barrier were also determined. Our results show that the activation energy for H2 dissociation on the decorated surface (0.052 eV) is much smaller than that of the pure surface (0.524 eV), elucidating that the activation condition of H2 on the pure ZrCo(110) is more severe than that on the Ti-doped surface. Particularly, Ti-decoration facilitates the H2 dissociation. Moreover, the re-desorption performance of the two dissociated H atoms is improved by lowering the energetic barrier from 1.798 eV (on the pure surface) to 1.315 eV (on the decorated surface). The calculations also reveal that decorating the surface with Ti eliminates the barrier for the into-bulk penetration of a hydrogen atom. Based on the local density of states, the Bader charge and differential charge density, as well as the influence of the Ti atom on topological properties were analyzed. Theoretical results presented in this study are generally in well accordance with the available theoretical and experimental data.