Real-space visualization of order-disorder transition in BaTiO.

Ferroelectricity in BaTiO was observed nearly 80 years ago, but the mechanism underlying its ferroelectric-paraelectric phase transition remains elusive. The order-disorder transition has been recognized as playing a critical role; however, the precise nature of the order parameter still remains under scrutiny, including the local dipole direction and the correlations above and below the Curie temperature. Using in situ scanning transmission electron microscopy, we directly map polar displacements in BaTiO across the ferroelectric-paraelectric phase transition, providing atomistic insights into an order-disorder mechanism. Atomic tracking reveals finite polar Ti displacements in the paraelectric phase where they manifest as random polar nanoregions. The displacements align along <111> direction in both the ferroelectric and paraelectric phases. The paraelectric-ferroelectric transition emerges from real-space correlations of the <111> polar Ti displacements. Our direct visualizations provide atomic insights into the order-disorder mechanism in the ferroelectric-paraelectric transition of BaTiO.