Lattice strain in epitaxial BaTiO3 thin films

We have investigated the out-of-plane lattice strain related to the ferroelectric transitions in epitaxial BaTiO3 (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 -> r or c -> r transitions, following the notations by Pertsev [Phys. Rev. Lett. 80, 1988 (1998)]. The orientations of the domains are consistent with theoretical predictions.