Formation, electronic structure, and defects of Ni substituted spinel cobalt oxide: A DFT+U study

Nickel substituted spinel cobalt oxide is a promising technological material with complex electronic and magnetic structures. Understanding these structures is important for improving the material's performance in various applications. We have carried out first-principles calculations on the formation, electronic properties, and defects of bulk NiCo₂O₄ using density functional theory (DFT) with on-site Hubbard U terms on the transition metal d states. Analysis of the electronic structure of Ni_xCo_3-xO₄ as a function of x = 0-1 shows that Ni acts as a p-type dopant in Co₃O₄, gradually transforming the minority spin channel from insulating to conducting. As a result, the inverse spinel NiCo₂O₄ (NCO) is found to have a ferrimagnetic half-metallic ground state with fractional valence on Ni and Co cations at tetrahedral sites (Td), in agreement with experimental observations. Projected densities of states confirm that the states around the Fermi energy originate from Ni and Co(Td) 3d states hybridized with oxygen 2p orbitals. The influence of two common defects, Ni ↔ Co(Td) exchanges and oxygen vacancies, on the structural and electronic properties has been also investigated. Our results are consistent with the experimental observation that intermediate structures between inverse spinel and normal spinel occur frequently in NCO. Oxygen vacancies are predicted to occur more frequently at sites coordinated to a larger number of Ni ions and found to have only minor effects on the conductivity and magnetic structure.