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${}_{\mathrm{C}}$) has a $c$-axis parameter of 4.64 Å and a tilting angle (β $=$ 88.5°) along the $a$ axis, while the high-temperature phase (M${}_{\mathrm{A}}$) has a $c$-axis parameter of 4.66 Å and a tilting angle (β $=$ 86.8°) along both of the $a$ and $b$ axes. We further show that samples undergoing the M${}_{\mathrm{C}}$–M${}_{\mathrm{A}}$ 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${}_3$.

• Received 26 June 2011

DOI:https://doi.org/10.1103/PhysRevB.85.014104