Correlations between polarization and structural information of supertetragonal PbTiO3

The huge axial ratio of supertetragonal perovskites can induce large polarization due to the coupling between the unit cell and ion displacements, which often results in higher Curie temperature or better ferroelectric properties. With the progress of vertical strain engineering, experimentally available supertetragonal PbTiO3 raises much interest in the relation between its intrinsic polarization and structural characteristics, such as the axial ratio and ion displacements that can be obtained with a high-resolution (scanning) transmission electron microscope nowadays. However, the interpretation and use of such obtained structural information are not without peril. Here, employing first-principles calculations, we report on the relationship between the polarization and the structural information for supertetragonal PbTiO3. Our investigation shows that, unlike normal PbTiO3, the strong sublinear correlation raises the difficulty to quantitatively estimate the spontaneous polarization for the axial ratio larger than 1.15. In addition, the relative displacement between cations (i.e., Pb and Ti) proves to be an unsuitable structural parameter to determine the spontaneous polarization, emphasizing the importance of obtaining the positions of anions, albeit presumably more difficult, along with cations.