  In ferroelectricity, atomic-scale dipole moments interact collectively to
produce strong electro-mechanical 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 put 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
sidewalk grows towards the bulk. We quantified the nucleation barrier and show
dominancy of mechanical interactions, 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.
