Chemical Bonding, Point Defects and Positron Lifetimes in FeSi2 from First-Principles Calculations

Abstract

The chemical bonding and formation energy of Fe and Si vacancies in α, β and γ-FeSi2 have been investigated using first-principles calculations. The positron lifetimes of those FeSi2 are also calculated. The bonding and antibonding bands in β-FeSi2 are completely separated, which leads to a gap at the Fermi level in β-FeSi2. In γ-FeSi2, the separation is not perfect and the Fermi level lines in the bonding band. Whereas the formation energy of a Fe vacancy in β-FeSi2 is higher than that of the other phases, it steeply decreases going from the Fe-rich to the Si-rich condition and shows the lowest value, about 0.4 eV, at the Si-rich limit. In α and γ-FeSi2, the Fe vacancy has a shorter positron lifetime than the Si vacancy. However, in β-FeSi2, positron lifetimes of the Fe and Si vacancies are close to each other. This result suggests that it may be difficult to identify vacancy sites in β-FeSi2 by positron lifetime measurement.