The polarization switching mechanism in polycrystalline ferroelectric/ferroelastic materials

Ferroelectrics find application in numerous devices and are considered as vital parts of emerging new areas, such as robotics, nanoelectronics and energy harvesting. The fundamental properties of these materials are intimately... [ view full abstract ]

Ferroelectrics find application in numerous devices and are considered as vital parts of emerging new areas, such as robotics, nanoelectronics and energy harvesting. The fundamental properties of these materials are intimately related to the unique ability to switch the polarization direction with electric fields. However, theoretical and experimental reports are inconsistent regarding the individual events that take place during switching. For example, theoretical models consider that switching in polycrystalline ferroelectric materials takes place by one event occurring at a characteristic switching time or over a distribution thereof, while experimental evidence indicates the presence of multiple steps.

To address this issue, we simultaneously measured macroscopic polarization and strain dynamics over a broad time range using high voltage pulses on a model polycrystalline ferroelectric Pb(Zr,Ti)O₃ system. Complemented by in situ time resolved diffraction and Landau free energy calculations the sequence of individual switching events could be revealed: initial non-180° domain wall movement from the poled state triggers the main switching phase due to an interplay between local electric and mechanical fields. In the main switching phase >60% of the polarization reversal occurs by 180° or synchronized strain-free non-180° switching events. This is followed by creep-like non-180° domain wall movement, whereby the polarization vector is parallel to the applied electric field. Field-dependent measurements of characteristic switching times allow the determination of the activation barriers of individual events, which are important for micromechanical simulations. Moreover, a strong time correlation of non-180° switching events in different grains was observed, which contradicts the general assumptions made in statistical models. These experimental results can thus help to further improve the theoretical models and provide new guidelines for targeted manipulation of individual switching processes and consequently functional properties of ferroelectrics.