Pressure-induced phase transitions of multiferroic BiFeO 3 have been investigated using synchrotron radiation X-ray diffraction with diamond anvil cell technique at room temperature. Present experimental data clearly show that rhombohedral (R3c) phase of BiFeO 3 first transforms to monoclinic (C2/m) phase at 7 GPa, then to orthorhombic (Pnma) phase at 11 GPa, which is consistent with recent theoretical ab initio calculation. However, we observe another peak at 2θ=7° in the pressure range of 5-7 GPa that has not been reported previously. Further analysis reveals that this reflection peak is attributed to the orthorhombic (Pbam) phase, indicating the coexisting of monoclinic phase with orthorhombic phase in low pressure range.
Keywords:BiFeO
1.INTRODUCTIONMultiferroics, which exhibit both ferroelectric and magnetic order in the same phase, have attracted great interests because of these unique properties [1,2]. Especially, the magnetic properties of multiferroics can be modulated by external electric fields and vice versa. Bismuth ferrite, BiFeO 3 (BFO) as a prototype multiferroic material, is perhaps the only compound with both magnetic and ferroelectric at ambient conditions [3,4]. In BFO, the 6s 2 lone pair electrons of Bi 3+ arouse the ferroelectricity until the temperature reaches T C =1100 K [5]. Spiral spin cycloid modulated G-type antiferromagnetic order with a periodicity of 62 nm occurs below the Neel temperature (T N =640K) [5,6]. Due to these promising physical properties, many experimental and theoretical efforts have been devoted to its structural characteristics [7] and the nature of magnetoelectric coupling [8,9]. At ambient conditions, BFO crystallizes in the rhombohedral structure with R3c (Z=161) space group, with c-axis parallel to the diagonals of the perovskite cube [10]. The Fe 3+ ion is coordinated by six oxygen atoms, forming an octahedral FeO 6 . Research of phase transitions under high pressures of BFO has been of long-standing interest [3,7,11,12]. Early literature has reported that no phase transition occurs in BFO until the high pressure reaches up to 70 GPa [13]. In contrast, some studies have demonstrated that BFO has undergone several structural phase transitions during the upstroke process. At high pressure above 10 GPa, Haumont et al.[12] and Guennou et al. [14] found that BFO transforms to the orthorhombic Pnma structure instead of others. While at low pressure, BFO exhibits a variety of phase transitions. For instance, by using x-ray diffraction (XRD) and far-infrared spectroscopy, Haumont et al. [12] found that BFO remains the rhombohedral structure up to 6.2 GPa, then changes to a monoclinic structure with space group C2/m. Nevertheless, in a more recent study, a phase transition from the polar rhombohedral R3c phase to the antipolar orthorhombic Pbam phase with antiferroelectric character of atomic displacements has been revealed at 3 GPa by using neutron powder diffraction [15]. Furthermore, Guennou et al. [14] have reported that three different phase transitio...
