Interactions of carbon–nitrogen and carbon–nitrogen–vacancy in α-Fe from first-principles calculations

Carbon and nitrogen are the most frequent foreign interstitial atoms in α-Fe. The presence of even a little amount of these impurities in interstitial positions has a drastic influence on the properties of steels. However, the physics of the C–N interaction remains unclear. Here we perform first-principles calculations based on the density functional theory to study the interactions of C–N and those with vacancy (□) in α-Fe. When C and N atoms are in neighbor octahedral sites, a hybridization around −8 eV of C 2p states with the N 2p states is observed. C and N atoms lie as far away from one another as possible. When a vacancy is introduced, it can attract two C atoms and a N atom, and a C–C–N–□ complex is formed. When the C and N atoms are the first nearest neighbors of the vacancy and stay in two neighbor octahedral sites, a covalent C–N bond is formed. And the peak of 2p states of the C atom’s density of states shifts to around −8 eV, where the peak of the N atom’s 2p states lies.