βphase andγ−βmetal-insulator transition in multiferroicBiFeO3

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 $\beta$ phase between 820 and 925 $(\pm 5)°\mathrm{C}$, and establish the existence range of the cubic $\gamma$ phase between 925 $(\pm 5)$ and 933 $(\pm 5)°\mathrm{C}$, contrary to all recent reports. We also report the refined ${\mathrm{Bi}}_2{\mathrm{O}}_3\text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$ phase diagram. The phase transition sequence rhombohedral-orthorhombic-cubic in bulk [monoclinic-orthorhombic-cubic in $\left(001\right)\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_3$ thin film] differs distinctly from that of $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_3$. The transition to the cubic $\gamma$ phase causes an abrupt collapse of the band gap toward zero (insulator-metal transition) at the orthorhombic-cubic $\beta \text{−}\gamma$ transition around $930°\mathrm{C}$. Our band structure models, high-temperature dc resistivity, and light absorption and reflectivity measurements are consistent with this metal-insulator transition.