Atomic structure of selenium incorporated into mackinawite unveiled by density functional theory

Selenium-79 is an important contributor to the long-term radiological hazards in deep geological repositories for high‑level radioactive waste. Investigating the environmental behavior of selenium (Se) is crucial for its safety assessment. In this study, the stable structures of Se incorporated in mackinawite, an iron sulfide mineral, were elucidated using density functional theory (DFT) calculations. The results indicate that it is important for Se atoms to be separated by a sufficient distance from one another. Furthermore, over the full concentration range, a symmetric structure at a Se atomic fraction of 0.5 was found to be the most stable.