<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>β</mml:mi></mml:math>phase and<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mi>γ</mml:mi><mml:mtext>−</mml:mtext><mml:mi>β</mml:mi></mml:mrow></mml:math>metal-insulator transition in multiferroic<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mi mathvariant="normal">Bi</mml:mi><mml:mi mathvariant="normal">Fe</mml:mi><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mn>3</mml:mn></mml:msub></mml:mrow></mml:math>

Abstract

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 $(\ifmmode\pm\else\textpm\fi{}5)\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$, and establish the existence range of the cubic $\ensuremath{\gamma}$ phase between 925 $(\ifmmode\pm\else\textpm\fi{}5)$ and 933 $(\ifmmode\pm\else\textpm\fi{}5)\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$, 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 $930\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$. Our band structure models, high-temperature dc resistivity, and light absorption and reflectivity measurements are consistent with this metal-insulator transition.