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AIP Advances, 2018
The electronic structures of α-BiFeO 3 are calculated by using a full-potential linearized-augmen... more The electronic structures of α-BiFeO 3 are calculated by using a full-potential linearized-augmented-plane-wave method. We employed the local-density approximation (LDA) with the modified Becke-Johnson (mBJ) exchange potential and the LDA + U method. The indirect (direct) bandgap of 2.24 (2.44) eV obtained by LDA + U method is in good agreement with an experiment, while the mBJ potential produces the indirect bandgap of 2.55 eV, and the direct bandgap is slightly larger than the indirect one. The discrepancy between the experimental x-ray spectra and the calculated Fe-3d and O-2p density of states were revealed to be due to the effects of the core hole. The core-hole effects are also responsible for the smaller bandgap in x-ray spectroscopy than the optical spectroscopy. The calculated valencecharge density and the bonding character obtained by LDA + U method also provides the stronger ionic character of the compound than the mBJ potential. Although the mBJ method is very efficient one, it is still very time consuming compared to the LDA + U method. The most suitable exchange-correlation potential for α-BiFeO 3 is the LDA + U. Therefore, it is better to use the LDA + U method for the electronicstructure calculations of BiFeO 3 compound not only for reducing the calculational time but also for better description of bandgaps and some physical properties. From the similar calculations carried out for transition-metal monoxide system it was found that the inadequacy of using the mBJ potential for the description of the localized 3d-states is rather universal.
Journal of the Korean Physical Society
Density-functional-theory-based calculations have been carried out to investigate the structural ... more Density-functional-theory-based calculations have been carried out to investigate the structural stability of bismuth ferrite (α-BiFeO3). α-BiFeO3 was generally observed to be in a hexagonal phase with the space group R3c. In a new experiment, however, several different crystal structures were suggested, and the triclinic phase (space group: P1) was claimed to be the most stable one. In order to confirm the claim theoretically, we carried out electronic-structure calculations for the various crystal structures suggested experimentally. Unlike the new experimental claim, we found that the hexagonal phase (R3c) had the lowest total energy. Furthermore, the hexagonal phase has a direct band gap of 0.87 eV. Even though this value is much smaller than the experimental value (1.3 eV) because of the notorious deficiency of the generalized-gradient approximation employed in this investigation, it is the closest one to the experimental one among the calculated band gaps of the investigated models. To understand the differences among different models, we investigated the band structure, density of states, and charge density. Along with the bonding process, the charge transfer was analyzed using the atoms-in-molecules theory. Based on this topological analysis of the bonding character, the evolution of the bonding strength with the critical points along the bonding trajectory and the valence charge in the atomic basins are presented quantitatively. The results show that the hexagonal phase has the strongest ionic character. Furthermore, the stability of our claimed model can be further assured by the bond ellipticity, which is a measure of the deviation of the charge distribution of a bond path from axial symmetry.
AIP Advances, 2018
The electronic structures of α-BiFeO 3 are calculated by using a full-potential linearized-augmen... more The electronic structures of α-BiFeO 3 are calculated by using a full-potential linearized-augmented-plane-wave method. We employed the local-density approximation (LDA) with the modified Becke-Johnson (mBJ) exchange potential and the LDA + U method. The indirect (direct) bandgap of 2.24 (2.44) eV obtained by LDA + U method is in good agreement with an experiment, while the mBJ potential produces the indirect bandgap of 2.55 eV, and the direct bandgap is slightly larger than the indirect one. The discrepancy between the experimental x-ray spectra and the calculated Fe-3d and O-2p density of states were revealed to be due to the effects of the core hole. The core-hole effects are also responsible for the smaller bandgap in x-ray spectroscopy than the optical spectroscopy. The calculated valencecharge density and the bonding character obtained by LDA + U method also provides the stronger ionic character of the compound than the mBJ potential. Although the mBJ method is very efficient one, it is still very time consuming compared to the LDA + U method. The most suitable exchange-correlation potential for α-BiFeO 3 is the LDA + U. Therefore, it is better to use the LDA + U method for the electronicstructure calculations of BiFeO 3 compound not only for reducing the calculational time but also for better description of bandgaps and some physical properties. From the similar calculations carried out for transition-metal monoxide system it was found that the inadequacy of using the mBJ potential for the description of the localized 3d-states is rather universal.
Journal of the Korean Physical Society
Density-functional-theory-based calculations have been carried out to investigate the structural ... more Density-functional-theory-based calculations have been carried out to investigate the structural stability of bismuth ferrite (α-BiFeO3). α-BiFeO3 was generally observed to be in a hexagonal phase with the space group R3c. In a new experiment, however, several different crystal structures were suggested, and the triclinic phase (space group: P1) was claimed to be the most stable one. In order to confirm the claim theoretically, we carried out electronic-structure calculations for the various crystal structures suggested experimentally. Unlike the new experimental claim, we found that the hexagonal phase (R3c) had the lowest total energy. Furthermore, the hexagonal phase has a direct band gap of 0.87 eV. Even though this value is much smaller than the experimental value (1.3 eV) because of the notorious deficiency of the generalized-gradient approximation employed in this investigation, it is the closest one to the experimental one among the calculated band gaps of the investigated models. To understand the differences among different models, we investigated the band structure, density of states, and charge density. Along with the bonding process, the charge transfer was analyzed using the atoms-in-molecules theory. Based on this topological analysis of the bonding character, the evolution of the bonding strength with the critical points along the bonding trajectory and the valence charge in the atomic basins are presented quantitatively. The results show that the hexagonal phase has the strongest ionic character. Furthermore, the stability of our claimed model can be further assured by the bond ellipticity, which is a measure of the deviation of the charge distribution of a bond path from axial symmetry.