The Effect of Degrees of Inversion on the Electronic Structure of Spinel NiCo₂O₄: A Density Functional Theory Study

In this study, electronic structure calculations and Bader charge analysis have been completed on the inverse, intermediate, and normal spinel structures of NiCo₂O₄ in both primitive and conventional cells, using density functional theory with Hubbard U correction. Three spinel structures have been computed in the primitive cell, where the fully inverse spinel, 50% intermediate spinel, and normal spinel can be acquired by swapping Ni and Co atoms on tetrahedral and octahedral sites. Furthermore, NiCo₂O₄ with different degrees of inversion in the conventional cells was also investigated, along with their doping energies. Confirmed by the assigned formal charges, magnetic moments, and decomposed density of state, our results suggest that the electronic properties of Ni and Co on the tetrahedral site can be altered by swapping Ni and Co atoms, whereas both Ni and Co on the octahedral site are uninfluenced. A simple and widely used model, crystal field theory, is also compared with our calculations and shows a consistent prediction about the cation distribution in NiCo₂O₄. This study analyzes the correlation between cation arrangements and formal charges, which could potentially be used to predict the desired electronic properties of NiCo₂O₄ for various applications.