Direct Observation of Large Atomic Polar Displacements in Epitaxial Barium Titanate Thin Films

The development of ferroelectric perovskite oxides having a controlled polarization direction is an ongoing and challenging topic of research. Here direct observation of large atomic polar displacements, which correspond to a polar density of ≈0.9 C m⁻² pointing upward, in an epitaxial BaTiO₃ film grown by molecular beam epitaxy on a SrTiO₃ substrate is reported. Aberration-corrected scanning transmission electron microscopy is used to map the polarization displacement with unit-cell resolution. Oxygen vacancies and other types of defects are examined and mapped using electron energy-loss near-edge structure analysis. The contributions from strain, strain gradient, and defects are quantitatively modeled in order to explain the large polarization. Calculations show that strain (through a defect dipole-enhanced polarization) creates the large atomic polar displacements, and strain gradient (through inverse Vegard electrochemical strain effect) compensates the polarization. These two effects may explain the preferred polarization direction and the anomalous flexoelectric effect in ferroelectric thin films.