Lattice Relaxation in Epitaxial BaTiO${}_3$ Thin Films

We have investigated the out-of-plane lattice relaxation related to the ferroelectric transitions in epitaxial BaTiO${}_3$ (BTO) films using synchrotron X-ray diffraction. Under either compressive strain or tensile strain, there is evidence for two structural phase transitions as a function of temperature. The transition temperature $T_C$ is a strong function of strain, which can be as much as 100 K above the corresponding $T_C$ in bulk. Under compressive strain, the tetragonality of BTO unit cell implies that the polarization of the first ferroelectric phase is out-of-plane, while under tensile strain, the polarization is in-plane. The transitions at lower temperature may correspond to the $aa\to r$ or $c\to r$ transitions, following the notations by Pertsev $\mathit{\text{et}}\mathit{\text{al}}$. The orientations of the domains are consistent with theoretical predictions.