The electronic, optical and ferroelectric properties of BiFeO₃ during polarization reversal: A first principle study

Highlights: • The nonlinear optical response of BiFeO₃ changes nonlinearly with polarization. • Nonlinear optical response could be used as criteria for the polarization reversal. • Analyzed the origin of the change of nonlinear optical response. • The hybridization of Bi 6s and O 2p leads to the ferroelectric displacement. - Abstract: Rhombohedral BiFeO₃ which has a large spontaneous polarization is a candidate material for ferroelectric random access memory (FeRAM). And the polarization reversal is the key factor for the application of FeRAM. Here, the electronic, linear and nonlinear optical properties, structure and its stability of BiFeO₃ during the continuous polarization reversal from reference cubic phase to ferroelectric R3c phase were systematically investigated by LSDA + U (local spin density approximation plus Hubbard U) and DFPT (density functional perturbation theory) method. We proposed that the nonlinear optical second order coefficients could be indicators to check the polarization reversal of BiFeO₃ and our result showed that the coefficient reach its maximum between 20% distortion and 40% distortion during the half path of the reversal. The relationship between the nonlinear optical second order coefficient and the Bader charge was also compared and we found the coefficient is also dependent on the Bader charge. In addition, the linear optical absorption coefficient was calculated; the obvious change in absorption peaks was observed during the transition. By comparing the crystal orbital of the reference cubic phase and distorted R3c phase we found that the hybridized Bi 6s and O 2p is the main reason that causes the structure distortion. The local phonon density of states proves that the Bi and O interaction is the origin of the instability of cubic phase BiFeO₃. The factor group analysis showed the change of Raman and IR modes during the transition