R3c-R3m Octahedron-tilting Transition in Rhombohedrally-distorted BiFeO_3 Multiferroics

Multiferroic BiFeO_3 (BFO) with the rhombohedral R3c symmetry is characterized by a clockwisecounterclockwise tilting (θ) of the FeO_6 octahedron unit along the threefold pseudo-cubic [111]_c direction. A phase transition associated with the disappearance of θ is conceivable at a high temperature as thermal energy tends to break up this antiferro-distortive ordering network along the [111]_c direction. Based on a high-resolution synchrotron X-ray diffraction study, we have found that the R3c BFO undergoes a transition to the R3m symmetry (with θ = 0) at ∼530 K, which is substantially lower than the antiferro-to-paramagnetic transition temperature of 643 K. The energy lowering caused by an antiferrodistortive R3c pairing is estimated to be 0.28 eV per unit cell.