Grain boundary segregation of nickel vacancies and space charge zone formation in NiO through interactions among Ni2+, O2−, and Ni3+

Defects like nickel vacancies (VNi″), holes (Ni3+) with different segregation tendencies to grain boundary (GB) can govern the properties of nickel oxide (NiO) – a promising material for energy applications. An atomistic model of non-stoichiometric NiO bicrystal with θ°001100 GB is developed to understand the segregation of VNi″, Ni3+ to GB from various ionic interactions. According to the model lattice positions available for VNi″ in GB possess lower attractive (from Ni2+-O2− interactions) and higher repulsive (from Ni2+-Ni2+ interactions) energies when compared with those in grain interior. This is due to the differences in these inter-ionic distances in GB and grain interior. Hence VNi″ segregate to GB. Contrarily, grain interior is more favorable for formation of Ni3+ in GB. These Ni3+ form a space charge zone adjacent to the GB. The model helps in understanding VNi″ segregation to GB and space charge zone formation.