Electronic structure and binding energies of hydrogen-decorated vacancies in Ni

The electronic structure, binding energies, and magnetic properties of Ni-containing vacancies and vacancy-hydrogen complexes have been studied using a first-principles all-electron self-consistent embedded-cluster model based on local-spin-density-functional theory. The results describe the properties of perfect ferromagnetic Ni metal correctly. The calculated local-spin magnetic moment at the nearest-neighbor site of the monovacancy is found to be 30% larger than the bulk value. This magnetic moment, however, is reduced significantly as hydrogen occupies the vacancy center. Calculations of binding energies of six hydrogen atoms moving along the octahedral directions from the vacancy center reveal that the magnetic moments at the nearest-neighbor Ni site continually decrease, eventually coupling antiferromagnetically to the bulk moment. This occurs when hydrogen atoms are displaced from the vacancy center by a distance of a₀ /2, where a₀ is the lattice constant. This is analogous to the antiferromagnetic coupling in NiO. The trapping of a six-hydrogen-atom complex inside a vacancy is found to be energetically favorable. The results are compared with a recent experiment where copious vacancy formation under high hydrogen pressure and temperature was observed.