The beta Phase of Multiferroic Bismuth Ferrite and its beta-gamma Metal-Insulator Transition

We show that epitaxial (001) thin films of multiferroic bismuth ferrite BiFeO3 are monoclinic at room temperature instead of tetragonal or Rhombohedral as reported earlier . We report a orthorhombic order-disorder beta-phase between 820C and 950C contrary to the earlier report. The transition sequence monoclinic-orthorhombic phase in (001)BiFeO3 thin film (rhombohedral-orthorhombic transition in single crystal) resembles that of BaTiO3 or PbSc1/2Ta1/2O3. The transition to the cubic $gamma$-phase causes an abrupt collapse of the bandgap toward zero (insulator-metal transition) at the orthorhombic-cubic beta-gamma transition around 950C. This transition is similar to the metal-insulator transition in Ba0.6K0.4BiO3.