Insight into the oxidation mechanism of the intermetallic compound NbAl3: A first-principles study

The microscopic mechanisms of the early stage oxidation of the NbAl₃(100) and NbAl₃(110) surfaces were studied by the first-principles method and thermodynamics calculations. First, the relationship between the relative surface energy of those two surfaces and Al chemical potential was investigated. Then, the binding energy per oxygen atom on those two surfaces at various adsorption sites was calculated and the preferential sites were determined. Finally, the surface phase diagrams of the NbAl₃(100) and NbAl₃(110) systems with different defects and different oxygen coverages were built. The results show that the defect-free NbAl₃(100) is the most stable surface before oxygen adsorption, while the NbAl₃(110) with Al antisite defects is the favorable configurations. Moreover, the adsorption of oxygen on those two surfaces is in favor of the surface segregation of Al and then leads to the formation of a complete Al surface layer at the initial stage of oxidation. Additionally, according to the density of states analysis, the significant orbital hybridizations between the O and Al in the ranges from −10 to 0 eV would lead to the formation of strong AlO bonding. Consequently, the oxygen atoms prefer to be adsorbed at the sites of Al-rich region.