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

TL;DR

This study uses density functional theory to analyze the electronic, magnetic, and bonding properties of Co3O4 spinel cobalt oxide, comparing different functionals and revealing hybridized covalent-ionic bonding characteristics.

Contribution

It provides a detailed DFT-based analysis of Co3O4's electronic structure and bonding, highlighting the effects of different exchange-correlation functionals.

Findings

GGA predicts Co3O4 as a semiconductor but underestimates the band gap.

GGA+U results align well with experimental band gap (~1.6 eV).

Bonding involves hybridized covalent and ionic components.

Abstract

The spinel cobalt oxide Co3O4 is a magnetic semiconductor containing cobalt ions in Co2+ and Co3+ 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 3 unpaired d electrons on the Co2+ 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 sp3d 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.