Strong Jahn-Teller effect at NiO4 tetrahedron in NiCo2O4 spinel

Abstract

The local structural distortion and electronic structure of the NiCo2O4 spinel are studied from first-principles calculations. It is found that very strong Jahn-Teller (J-T) distortion occurs to the NiO4 tetrahedron and a large energy gain is obtained, leading to a substantial elongation of the lattice and a cubic to tetragonal phase transition. Further analysis shows that the Ni-3d states are split by not only the broken Td symmetry of the NiO4, chemical bonding interactions also exist between half-occupied dyz and dxz orbitals, which further lower the energy levels of the occupied dyz and dxz states in the spin-up channel. As a result, the energy gains from the J-T distortion are obtained to be 0.53 eV, 0.84 eV, and 0.83 eV per NiCo2O4 formula unit for using LDA, PBE and PW91 functionals, respectively.