Strain-driven phase boundaries in BiFeO3thin 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 BiFeO₃ (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 (MC) has a c-axis parameter of 4.64 {} and a tilting angle ({β} $=$ 88.5^∘{}) along the a axis, while the high-temperature phase (MA) 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 MC--MA 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 BiFeO₃.