Effect of Stoichiometry on the Structure and Polarization of BaTiO₃

Abstract Barium titanate (BaTiO₃) is a material of interest for photonic device applications due to its strong optical non‐linearity. However, BaTiO₃‐based devices have not found widespread adoption, in part due to the challenges associated with synthesizing high quality thin‐films. Here, high‐resolution scanning transmission electron microscope (STEM) imaging is used to investigate the atomic structure of both on‐ and off‐stoichiometric BaTiO₃ synthesized by molecular beam epitaxy (MBE). This investigation reveals an asymmetry in the way the BaTiO₃ atomic lattice accommodates off‐stoichiometry growth and unveils features beyond what is expected from diffraction or surface characterization techniques. Excess titanium incorporates into the BaTiO₃ lattice to form pervasive defects despite titanium‐rich films having a low surface roughness and high‐quality appearance in diffraction. Excess barium forms a rough, water‐soluble surface layer but does not significantly impact the quality of the BaTiO₃ lattice. STEM is used to map titanium atom displacement in real‐space. The average displacement distance is 30–60 pm in the strained thin‐films, higher than the <20 pm displacement in bulk BaTiO₃. Additionally, the titanium atom displacement direction deviates from the c‐axis of the unit cell, which may have implications for the material's electro‐optic tensor and thus for electro‐optic device design.