First-principles calculations of hydrogen interactions with nickel containing a monovacancy and divacancies

Density functional theory calculations were used to study the vacancy and hydrogen interaction behavior in nickel, through calculation of the energetics and electronic interactions. Divacancy interactions in the nickel bulk depend on the position of the vacancies, where 1NN sites have an attractive interaction, 2NN sites have a repulsive interaction and 3NN sites have almost no interaction. Hydrogen-vacancy interactions have binding energies of 0.51 eV and 0.41 eV for a hydrogen atom at the shared octahedral 'Os' site of a 2NN divacancy and 1NN divacancy, respectively. The calculated energy is in good agreement with previous observations. A monovacancy in nickel can trap up to six hydrogen atoms at the nearest octahedral site. However, a divacancy can accommodate almost four times as many hydrogen atoms as a monovacancy. 1NN octahedral sites can strongly bond with the hydrogen atoms, while hydrogen atoms at 2NN octahedral sites show a noticeably decreased segregation energy; however, the structures are still stable. A vacancy can significantly modify the charge density of the lattice. Hydrogen atoms at octahedral sites receive electrons from the NN metal atoms, forming bonds. As more hydrogen atoms are added, the isosurface is reduced, and there are fewer available optimal-density sites to accommodate additional hydrogen. Hydrogen at the 'Os' site of a 2NN divacancy receives more electrons from the nearest metal atoms, and as a result, a strong interaction occurs. A divacancy can trap almost four times as many hydrogen atoms as a monovacancy, leading to an increase in the hydrogen content in the metal.