Revealing Morphotropic Phase Boundary and Phase Transition Behavior in Strained BiFeO3 Thin Films

As the advance of technology, devices with multi-function have dictated the material design. Complex oxide materials, having the spatial variation of electron charge and lattice degree of freedom at interface, have attracted wide attention because of their various properties. Recently, strained bismuth ferrite (BiFeO₃, BFO) thin film is the most promising multifunctional material due to its spontaneous magnetic, ferroelectric and unique electromechanical properties at room temperature. In this study, we have investigated the temperature-dependent structural and strain-driven phase transition in epitaxial BFO thin films deposited on LaAlO₃ substrates. Atomic force microscopy (AFM) and X-ray diffraction spectroscopy (XRD) demonstrated that the tetragonal-like phase (T-phase) and rhombohedral-like phase (R-phase) were involved in the strain BFO thin films. High-resolution transmission electron microscopy (HRTEM) images show that the T-phase BFO has a large c-axis parameter of ~4.65 Å, whereas R-phase BFO has a relatively small c-axis parameter of ~4.05 Å. We further find that the tilt angle between R/T phases is about 4.65° along the a-axis. Using in-situ TEM, we observed that the BFO thin film exhibited R→T phase transition with increasing temperature. Upon increasing the temperature from room temperature to 200℃, we found a significant increase in the width of T-phase region with the broader morphology phase boundary (MPB). Interestingly, upon further increasing the temperature to 250~300℃, the MPB disappears gradually with the change of tilt angle. Eventually, only T-phase BFO would be preserved at high temperature. Our study directly demonstrates the detail of structural changes with temperature effect, which may provide important information for material design and promising multifunctional applications in future technology.