Absence of low-temperature phase transitions in epitaxial ${\mathrm{BaTiO}}_3$ thin films

We have studied phase transitions in epitaxial ${\mathrm{BaTiO}}_3$ thin films by Raman spectroscopy. The films are found to remain in a single ferroelectric phase over the temperature range from 5 to 325 K. The low-temperature phase transitions characteristic of bulk ${\mathrm{BaTiO}}_3$ (tetragonal-orthorhombic-rhombohedral) are absent in the films. X-ray diffraction shows that the ${\mathrm{BaTiO}}_3$ films are under tensile strain due to the thermal expansion mismatch with the buffer layer. A phase-field calculation of the phase diagram and domain structures in ${\mathrm{BaTiO}}_3$ thin films predicts, without any priori assumption, that an orthorhombic phase with in-plane polarization is the thermodynamically stable phase for such values of tensile strain and temperature, consistent with the experimental Raman results.