Full-Potential Study of the Electronic and Optical Properties of the Transparent Oxide ZnCo2O4 by Use of PBE and TB-mBJ Potentials

The electronic and optical properties of ZnCo2O4 spinel have been investigated by use of the full-potential linearized augmented plane wave method based on density functional theory. Calculations were performed by use of the alternative form of the generalized gradient approximation proposed by Perdew, Burke, and Ernzerhof (GGA-PBE) and by use of orbital-independent Tran-Blaha-modified Becke-Johnson (TB-mBJ) potentials as coupled with GGA. The optimized unit cell length, a (0), and internal parameter, u, were in agreement with experimental data. The band gap obtained by use of the TB-mBJ scheme is a significant improvement over the PBE value and is close to the experimental data. Our calculations of density of states (DOS) show that more pronounced splitting of Co-d states is responsible for the larger band gap of ZnCo2O4 calculated by use of the TB-mBJ scheme. Results for the DOS show that valence band dispersion is reduced in the TB-mBJ scheme compared with the PBE scheme. Optical properties were calculated for the energy range 0-14 eV. The calculated reflectivity stays low until 3.5 eV, which is consistent with the energy gap in ZnCo2O4. The results were analyzed on the basis of band-to-band transitions. We also report the frequency-dependent refractive index, n(omega), and the extinction coefficient, k(omega), of ZnCo2O4 obtained by use of the PBE and TB-mBJ schemes.