First-principles study of beryllium thermodynamics and clustering mechanism in molybdenum: Effects of vacancies and self-interstitial atoms

• A single Be atom preferentially occupies an octahedral interstitial site (OIS) rather than a tetrahedral interstitial site (TIS), and multiple interstitial Be atoms tend to aggregate to form Be n clusters. • The physical origin of the binding energy between Be atoms in Mo is elucidated, providing a clear explanation of our computational results. • Vacancies and self-interstitials (SIAs) significantly enhance Be aggregation, and the optimal configuration for Be clustering in Mo has been identified, with particular attention given to the effects of vacancies and SIAs. • This study reveals an underlying clustering mechanism for the formation of Be-rich regions in Mo. • The concentrations of both Be n clusters and Be n V clusters increase as the temperature rises, suggesting that elevated temperatures facilitate their formation in Mo. The clustering behavior of beryllium (Be) following plasma irradiation is of particular significance for molybdenum (Mo) in future fusion devices. Using first-principles calculations combined with thermodynamic models, the optimal configuration for Be clustering in Mo has been clearly determined, with a particular focus on the effects of vacancies and self-interstitials (SIAs). As the initial form of nucleation, the physical origin of Be-Be pair binding energy in Mo has been shown to be primarily dominated by the charge density at their location. Based on all of our computational results, a potential clustering mechanism for the formation of Be-rich regions in Mo is proposed: Be atoms first aggregate at interstitial sites, forming Be n clusters. When the number of Be atoms reaches six, they form an approximately “octahedral” structure, displacing a central Mo atom and generating a Be 6 V cluster and an SIA. They act as nucleation sites that continue to attract more Be atoms, growing into larger Be n V and Be n -SIA clusters. As the Be n -SIA clusters expand, excess Be atoms displace more Mo atoms, creating additional SIAs and vacancies and further propagating the formation of Be n -SIA and Be n V clusters. This cascading process ultimately results in the development of Be-rich regions within Mo. Our results provide significant data support for advancing Mo as a primary mirror material and also offer valuable theoretical insights into the aggregation behavior of impurities in metals under irradiation conditions.