Surface Nucleation of the Paraelectric Phase in Ferroelectric BaTiO₃: Atomic Scale Mapping

In ferroelectricity, atomic-scale dipole moments interact collectively to produce strong electromechanical coupling and switchable macroscopic polarization. Hence, the functionality of ferroelectrics emerges at a solid-solid phase transformation that is accompanied by a sudden disappearance of an inversion symmetry. Much effort has been made to understand the ferroelectric transition at the polarization length scale. Nevertheless, the dipole-moment origin of ferroelectricity has remained elusive. Here, we used variable-temperature high-resolution transmission electron microscopy to reveal the dipole-moment dynamics during the ferroelectric-to-paraelectric transition. We show that the transition occurs when paraelectric nuclei of the size of a couple of unit cells emerge near the surface. Upon heating, the cubic phase grows sidewalk toward the bulk. We quantified the nucleation barrier and show dominancy of mechanical interactions over electrical interaction, helping us demonstrate similarities to predictions of domain nucleation during electric-field switching. Our work motivates dynamic atomic-scale characterizations of solid-solid transitions in other materials.