High temperature phase transitions in BiFeO3

The high temperature phases of BiFeO3 have courted much controversy with many conflicting structural models reported, in particular for the paraelectric ?-phase. High temperature powder neutron diffraction (PND) experiments indicate that the ferroelectric (R3c) ?-phase transforms to the paraelectric ?-phase at approximately 820°C via a first order phase transition. We demonstrate that this phase is unambiguously orthorhombic, adopting the GdFeO3 structure-type with a space group Pbnm. On further heating BiFeO3 undergoes another first order phase transition (?-?) at approximately 930°C which is marked by a discontinuous decrease in cell volume consistent with an insulator-metal transition. Close inspection of the PND data show no evidence of any symmetry change, with the postulated ?-phase remaining orthorhombic Pbnm. In addition we present PND and impedance spectroscopy data for BiFeO3 which suggest that the so-called 'Polomska' transition observed by some authors at approximately 185°C is not intrinsic.