Strain-driven phase boundaries in BiFeO₃ thin films studied by atomic force microscopy and x-ray diffraction

We report a detailed study on the strain-driven phase transition between the tetragonal-like and rhombohedral-like phases in epitaxial BiFeO ₃ (BFO) thin films which focuses on their structural nature, thermodynamic stability, and ferroelectric/piezoelectric properties. We first show that the tetragonal-like phase, which has a large c/a ratio (∼1.2), in the compressively strained BFO is thermodynamically more favorable at high temperature and high strain state (small thickness). We also report a phase transition between two monoclinic phases at 150°C. The two monoclinic phases are differentiated by their c-axis parameters and tilting angles: The low-temperature phase (M _C ) has a c-axis parameter of 4.64 and a tilting angle (β = 88.5°) along the a axis, while the high-temperature phase (M _A ) has a c-axis parameter of 4.66 and a tilting angle (β = 86.8°) along both of the a and b axes. We further show that samples undergoing the M _C -M _A phase transition exhibit ferroelectric polarization rotation and piezoelectric enhancement. Our findings directly unveil the close links between structural changes, polarization rotation, and large piezoelectricity at morphotropic phase boundaries in BiFeO ₃ .