First-principles calculations to investigate the Electronic and Optical Properties of Hexagonal, Triclinic, and Monoclinic Structures of α-BiFeO₃

The electronic structures and optical properties of hexagonal (R3c), triclinic (P1), and monoclinic (R3) structures of the strongly correlated α-BiFeO₃ (BFO) systems are presented by adopting a full-potential linearized augmented plane-wave method. For an accurate bandgap, the exchange–correlation functional called the corrected local density of Hubbard (LDA + U) method and modified Becke-Johnson (mBJ) exchange potential are used. We performed several theoretical calculations to investigate the electronic and optical properties of BFO with the R3c space group only. Therefore, this study focuses on elucidating the physical properties of other space groups such as P1 and R3 of BFO. This study obtained indirect bandgaps of 2.24 and 2.55 eV for R3c, 2.3 and 2.5 eV for P1, and 2.15 and 2.45 eV for R3 obtained by LDA + U and mBJ methods, respectively. Only the direct bandgap (2.44 eV) for R3c, acquired by the LDA + U, perfectly matches the experimental absorption measurement of single-crystal BFO (2.4 eV). For the mBJ potential, for all BFO structure types, the direct bandgaps are slightly larger than the indirect bandgaps. The LDA + U spectrum features of the imaginary part of the dielectric function for the hexagonal and triclinic structure types are in better agreement with the experimental findings than those of the mBJ method. Although the BFO structures have indirect and direct bandgaps, the direct optical absorption of the LDA + U was not significant for the R3 structure. Although the mBJ method is highly efficient compared to the most accurate GW method, it is time-consuming compared to the LDA + U method. Therefore, LDA + U is considered inexpensive and more reliable than mBJ in better understanding optoelectronic properties. It is, therefore, preferable to use the LDA + U method for BFO electronic structural calculations to reduce the calculation time and obtain a better description of the bandgaps and some physical properties, particularly optical properties.