β phase and γ-β 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 β phase between 820 and 925 (±5)°C , and establish the existence range of the cubic γ phase between 925 (±5) and 933 (±5)°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 γ phase causes an abrupt collapse of the band gap toward zero (insulator-metal transition) at the orthorhombic-cubic β-γ transition around 930°C . Our band structure models, high-temperature dc resistivity, and light absorption and reflectivity measurements are consistent with this metal-insulator transition.