Strong Jahn-Teller effect at NiO4 tetrahedron in NiCo2O4 spinel

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 T-d symmetry of the NiO4, chemical bonding interactions also exist between half-occupied d(yz) and d(xz) orbitals, which further lower the energy levels of the occupied d(yz) and d(xz) 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. (C) 2019 Elsevier B.V. All rights reserved.