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 stud ied using a first-principles all-electron self-consistent embedded-clu ster model based on local-spin-density-functional theory. The results describe the properties of perfect ferromagnetic Ni metal correctly. T he 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 occupi es the vacancy center. Calculations of binding energies of six hydroge n atoms moving along the octahedral directions from the vacancy center reveal that the magnetic moments at the nearest-neighbor Ni site cont inually decrease, eventually coupling antiferromagnetically to the bul k moment This occurs when hydrogen atoms are displaced from the vacanc y center by a distance of a(0)/2, where a(0) is the lattice constant. This is analogous to the antiferromagnetic coupling in NiO. The trappi ng of a six-hydrogen-atom complex inside a vacancy is found to be ener getically favorable, The results are compared with a recent experiment where copious vacancy formation under high hydrogen pressure and temp erature was observed.