We have investigated the out-of-plane lattice relaxation 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 TC is a strong function of strain, which can be as much as 100 K above the corresponding TC 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 \textit{et al}. The orientations of the domains are consistent with theoretical predictions.
@article{arxiv.cond-mat/0511518,
title = {Lattice Relaxation in Epitaxial BaTiO$_3$ Thin Films},
author = {Feizhou He and Barrett O. Wells},
journal= {arXiv preprint arXiv:cond-mat/0511518},
year = {2007}
}