Effects of Hydrogen Atoms on Vacancy Formation at fcc Fe (111) Surfaces

We have investigated effects of hydrogen atoms on vacancy formation at fcc Fe (111) surfaces. To calculate hydrogen adsorption properties and vacancy formation energy, we performed first principles calculations based on density functional theory with generalized gradient approximation. We considered fcc Fe (111) surfaces with monoatomic vacancies in second and third surface layers. We constructed exhaustive potential energy surfaces of hydrogen atoms in the vicinity of monoatomic vacancies, and obtained the most stable adsorption sites and corresponding adsorption energy. We found that hydrogen atoms can stably adsorb in the vicinity of monoatomic vacancies, while hydrogen atoms cannot be stable in the fcc Fe (111) subsurfaces without monoatomic vacancies. We also clarified that the vacancy formation energy is reduced by containing hydrogen atoms in the vicinity of vacancy sites. These results indicate that hydrogen atoms and vacancies can stabilize each other, which results in superabundant vacancies of fcc Fe (111) in hydrogen atmosphere.