New insight into the interaction between divacancy and H/He impurity in Ti3AlC2 using first-principles studies.

First-principles calculations have been conducted to investigate the interaction between vacancy defects and H/He impurity in Ti₃AlC₂. The formation energies of monovacancy and divacancy have been calculated. It is found that Al monovacancy (V_Al), Al divacancy (2V_Al–Al), and the divacancy composed of Al and C atoms (2V_Al–C) are most easily formed in all vacancies. In addition, the interactions between multiple vacancies are weak. The formation of vacancy is relatively independent and not affected by other vacancies. The configurations and energies of H–mV (m = 0, 1, 2) complexes have been studied to assess the energetically favorable sites for H atoms. Within pre-existing V_Al or 2V_Al–Al, the most favorable site for H atoms is the I_tetr-2 site, but the H atom tends to occupy the I_oct-4 site within 2V_Al–C. The formation energies of the secondary vacancy defect nearest to an Al vacancy or C vacancy are significantly influenced by H impurity content. H clusters trapped in a primary Al vacancy can promote the formation of vacancy and prefer to form platelet-like bubbles parallel to the Al plane, while H clusters trapped in a primary C vacancy have higher probability to form spherical ones. The 2V_Al–Al and 2V_Al–C divacancies exhibit stronger H trapping ability than monovacancy. The 2V_Al–Al divacancy could capture up to seven H atoms, and 2V_Al–C could capture six H atoms. Meanwhile, the He–2V_Al–Al complex could only capture four H atoms to form H–He hybridized bubbles, and He impurities effectively suppress further aggregation of H atoms. The present results provide microstructural images of nH–mV and nH–He–mV complexes as well as the evolution progress of H bubbles in Ti₃AlC₂, which is especially helpful for us to understand the behavior of H/He in Ti₃AlC₂ under irradiation. [ABSTRACT FROM AUTHOR]