$\beta$ phase and $\gamma-\beta$ metal-insulator transition in multiferroic BiFeO$_3$
R. Palai, R. S. Katiyar, H. Schmid, P. Tissot, S. J. Clark, J. Robertson, S. A. T. Redfern, G. Catalan, and J. F. Scott

{\sf We report on extensive experimental studies on thin film, single crystal, and ceramics of multiferroic bismuth ferrite \bfo\, using differential thermal analysis, high temperature polarized light microscopy, high-temperature and polarized Raman spectroscopy, high-temperature X-ray diffraction and dc conductivity, optical absorption and reflectivity and domain imaging, and show that epitaxial (001) thin films of \bfo\ are clearly monoclinic at room temperature, in agreement with recent synchrotron studies but in disagreement with all other earlier reported results. We report an {\it orthorhombic} order-disorder $\beta$-phase between 820 and 925 ($\pm$5)\dc\ and establish the existence range of the cubic $\gamma$-phase between 925 ($\pm$5)\dc\ and 933 ($\pm$5)\dc\ , contrary to all recent reports. We also report the refined Bi$_{2}$O$_{3}$-Fe$_{2}$O$_{3}$ phase diagram. The phase transition sequence rhombohedral-orthorhombic-cubic in bulk (monoclinic-orthorhombic-cubic in (001)\bfo\ thin film) differs distinctly from that of \bto. 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 930\dc. Our bandstructure models, high temperature dc resistivity, and light absorption and reflectivity measurements are consistent with this metal-insulator transition.}

© 2008 The American Physical Society.