Ferroelectric Domain Structures in Low-Strain BaTiO₃

Epitaxial strain in ferroelectric films offers the possibility to enhance the piezoelectric performance utilizing low crystal symmetries and high density of domain walls. Ferroelectric BaTiO₃ has been predicted to order in a variety of phases and domain configurations when grown under low strain on low mismatched substrates, but little experimental evidence of that region of the phase diagram exist. Here, epitaxial BaTiO₃ thin films are grown on NdScO₃ substrates under ≈0.1% strain. A monoclinic ca₁/ca₂ phase, with 90° periodic in-plane domain configuration and small additional out-of-plane component of polarization, is stabilized at room temperature and investigated using piezoelectric force microscopy and X-ray diffraction. Above 50 °C, this phase is transformed into an a/c phase with alternating in-plane and out-of-plane polarizations and forming zigzag domain walls between up-polarized and down-polarized superdomains. Both types of domain patterns are highly anisotropic, giving rise to very long domain walls. Above 130 °C, the paraelectric phase is observed. The occurrence of a phase transition close to room temperature, a low symmetry ca₁/ca₂ phase, and the formation of periodic domains make of this material a promising candidate for high piezoelectric response.