Ferroelectric single crystals had been widely applied for medical imaging, actuation, and sensor technologies due to their piezoelectric effect. However, fundamental questions regarding the microscopic origin of ferroelectric phase transitions are still under debate despite many past efforts. Here, we demonstrate the energy-filtered scanning convergent beam electron diffraction (EF-SCBED) technique that can be simultaneously used for (1) the identification of polarization domains, (2) a determination of the local crystal symmetry within a single domain, and (3) identification of structural distortions.
Results from SCBED show that the crystal symmetry is not homogeneous; regions of few tens nm retaining almost perfect symmetry are interspersed in regions of lower symmetry. The highly symmetric regions confirm the acentric tetragonal, orthorhombic, and rhombohedral symmetry for the ferroelectric phases of BaTiO3 at different temperatures, which is consistent with the displacive model of ferroelectric phase transitions in BaTiO3. On the other hand, the observed nanoscale symmetry fluctuations are consistent with the predictions of order-disorder phase transition mechanism. For future work, the SCBED-based techniques, quantitative SCBED (SQCBED) and rocking SCBED (SRCBED), will be further developed for simultaneous identification of polarization, domain wall structures, crystal potential, and chemical bonding.