High Pressure Raman Investigation of BiFeO₃

BiFeO₃ is one of the most promising multiferroics that exhibits both magnetic and ferroelectric properties above room temperature.The room-temperature structure of BiFeO₃ is a highly rhombohedrally distorted perovskite with space group R3c.In this study, we prepared BiFeO₃ powder under high temperature and high pressure using muti-anvils, and investigated the phase transition of BiFeO₃ in the 0-44 GPa range combinng with Raman spectrum.Upon compression, the low-frequency Raman modes of BiFeO₃ shift to higher angles and become broadening, and the vibrational modes at 145, 177 and 231 cm^-1 begin to decrease.The first phase transition takes place at the pressure of about 5 GPa, shown by the disappearance of the mode at 145 cm^-1 and the emergence of the mode at 217 cm^-1.The second phase transition is indicated by the emergence of the mode at 340 cm^-1 and the disappearance of the mode below 200 cm^-1 at 11 GPa.After this phase transition, the structure of BiFeO₃ transfers to orthorhombic phase Pnma.The third phase transition takes place at the pressure of 38 GPa, all the Raman modes including the mode at 340 cm^-1 disappear, and the structure of BiFeO₃ may transferr to cubic or higher symmetry orthorhombic crystals.