COMPUTER SIMULATION OF THE ELECTRONIC STRUCTURE OF HEMATITE
Abstarct. The results of ab initio computer modeling of the electronic structure of bulk rhomboedral hematite (a-Fe<sub>2</sub>O<sub>3</sub>) are presented. The spectra of total and local partial densities and bandstructures for both spin directions have been calculated using the full-potential linearized augmented plane wave (FP-LAPW) method together with conventional and modified density-functional approaches at LSDA, GGA, LSDA + U and GGA + U levels. Using conventional LSDA and GGA functional we obtain following results. The magnetic moments of the Fe atoms and the band gap are too small. Also, the character of the gap contradicts the accepted charge-transfer character. Analysis of the density of states confirms the strong hybridization between Fe 3d and O 2p states at the top of the valence band. The DFT + U calculations were performed in order to take into account the strong on-site Coulomb interaction between 3d-electrons of Fe atoms. Introducing a Hubbard like term in the density functional results in strongly improved values for magnetic moments, band gap as well as in better agreement of the calculated density of states with experimental PES spectra. We found that with increasing parameter U (at LSDA and GGA levels alike) the size of the band gap and the local magnetic moments of Fe increase. Best overall agreements with respect to experimental values of the band gap, magnetic moments and to experimental photoemission spectra of hematite, is achieved for U = 5 eV. Also, an important result of this study is the fundamental change in the semiconducting gap from a d-d exchange gap to an O 2p-Fe 3d charge-transfer gap, together with change of the highest occupied valence states from strongly hybridized O 2p -Fe 3d to almost pure O 2p character.