Electronic structure and bonding properties of cobalt oxide in the spinel structure

The spinel cobalt oxide Co₃O₄ is a magnetic semiconductor containing cobalt ions in Co^2+ and Co^3+ oxidation states. We have studied the electronic, magnetic, and bonding properties of Co₃O₄ using density functional theory (DFT) at the Generalized Gradient Approximation (GGA), GGA+U, and PBE0 hybrid functional levels. The GGA correctly predicts Co₃O₄ to be a semiconductor but severely underestimates the band gap. The GGA+U band gap (1. 96 eV) agrees well with the available experimental value (1. 6 eV), whereas the band gap obtained using the PBE0 hybrid functional (3. 42 eV) is strongly overestimated. All the employed exchange-correlation functionals predict three unpaired d electrons on the Co^2+ ions, in agreement with crystal field theory, but the values of the magnetic moments given by GGA+U and PBE0 are in closer agreement with the experiment than the GGA value, indicating a better description of the cobalt localized d states. Bonding properties are studied by means of maximally localized Wannier functions (MLWFs). We find d-type MLWFs on the cobalt ions, as well as Wannier functions with the character of sp^3d bonds between cobalt and oxygen ions. Such hybridized bonding states indicate the presence of a small covalent component in the primarily ionic bonding mechanism of this compound.