Influence of Fe Vacancy on the Bonding Properties of γ-Fe (111)/α-Al₂O₃ (0001) Interfaces: A Theoretical Study

Here, the effects of Fe vacancy defects on the bonding properties of γ-Fe (111)/α-Al₂O₃ (0001) interfaces are studied in depth at the atomic and electronic levels using first-principles calculations. The first (V₁), second (V₂), third (V₃), and fourth (V₄) layers of vacancy structures within the Fe substrate, as well as the ideal Fe/Al₂O₃ interface structure, are proposed and contrasted, including their thermodynamic parameters and atomic/electronic properties. The results demonstrate that the presence of vacancies in the first atomic layer of Fe deteriorates the interfacial bonding strength, whereas vacancies situated in the third layer enhance the interfacial bonding strength. The effect of vacancy beyond the third layer becomes negligible. This occurs mainly because vacancy defects at different positions induce the relaxation behavior of atoms, resulting in bond-breaking and bond-forming reactions at the interface. Following that, the formation process of vacancies can cause the transfer and rearrangement of the electrons at the interface. This process leads to significant changes in the charge concentration of the interfaces, where V₃ is the largest and V₁ is the smallest, indicating that the greater the charge concentration, the stronger the bonding strength of the interface. Furthermore, it is discovered that vacancy defects can induce new electronic orbital hybridization between Fe and O at the interface, which is the fundamental reason for changes in the properties of the interface. Interestingly, it is also found that more electronic orbital hybridization will strengthen the bonding performance of the interface. It seems, then, that the existence of vacancy defects not only changes the electronic environment of the Fe/Al₂O₃ interface but also directly affects the bonding properties of the interface.