beta phase and gamma-beta metal-insulator transition in multiferroic BiFeO3

We report on extensive experimental studies on thin film, single crystal, and ceramics of multiferroic bismuth ferrite BiFeO3 using differential thermal analysis, high-temperature polarized light microscopy, high-temperature and polarized Raman spectroscopy, high-temperature x-ray diffraction, dc conductivity, optical absorption and reflectivity, and domain imaging, and show that epitaxial (001) thin films of BiFeO3 are clearly monoclinic at room temperature, in agreement with recent synchrotron studies but in disagreement with all other earlier reported results. We report an orthorhombic order-disorder beta phase between 820 and 925 (+/- 5) degrees C, and establish the existence range of the cubic gamma phase between 925 (+/- 5) and 933 (+/- 5) degrees C, contrary to all recent reports. We also report the refined Bi2O3-Fe2O3 phase diagram. The phase transition sequence rhombohedral-orthorhombic-cubic in bulk [monoclinic-orthorhombic-cubic in (001)BiFeO3 thin film] differs distinctly from that of BaTiO3. The transition to the cubic gamma phase causes an abrupt collapse of the band gap toward zero (insulator-metal transition) at the orthorhombic-cubic beta-gamma transition around 930 degrees C. Our band structure models, high-temperature dc resistivity, and light absorption and reflectivity measurements are consistent with this metal-insulator transition.