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 $mathrm{Bi}mathrm{Fe}{mathrm{O}}_{3}$ 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 $mathrm{Bi}mathrm{Fe}{mathrm{O}}_{3}$ 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 $ensuremath{beta}$ phase between 820 and 925 $(ifmmodepmelsetextpmfi{}5)phantom{rule{0.2em}{0ex}}ifmmode^circelsetextdegreefi{}mathrm{C}$, and establish the existence range of the cubic $ensuremath{gamma}$ phase between 925 $(ifmmodepmelsetextpmfi{}5)$ and 933 $(ifmmodepmelsetextpmfi{}5)phantom{rule{0.2em}{0ex}}ifmmode^circelsetextdegreefi{}mathrm{C}$, contrary to all recent reports. We also report the refined ${mathrm{Bi}}_{2}{mathrm{O}}_{3}text{ensuremath{-}}{mathrm{Fe}}_{2}{mathrm{O}}_{3}$ phase diagram. The phase transition sequence rhombohedral-orthorhombic-cubic in bulk [monoclinic-orthorhombic-cubic in $(001)mathrm{Bi}mathrm{Fe}{mathrm{O}}_{3}$ thin film] differs distinctly from that of $mathrm{Ba}mathrm{Ti}{mathrm{O}}_{3}$. The transition to the cubic $ensuremath{gamma}$ phase causes an abrupt collapse of the band gap toward zero (insulator-metal transition) at the orthorhombic-cubic $ensuremath{beta}text{ensuremath{-}}ensuremath{gamma}$ transition around $930phantom{rule{0.2em}{0ex}}ifmmode^circelsetextdegreefi{}mathrm{C}$. Our band structure models, high-temperature dc resistivity, and light absorption and reflectivity measurements are consistent with this metal-insulator transition.