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Stability of the crystal structure of α-BiFeO3

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.