Decoding Thermal Depolarization Temperature in Bismuth Ferrite-Barium Titanate Relaxor Ferroelectrics with Large Strain Response.

Bismuth ferrite-barium titanate (BF-BT) ferroelectrics attract attention due to their multifunctional properties and potential applications in high-temperature piezodevices. The thermal depolarization temperature (Td) of BF-BT-based relaxor ferroelectrics has a close correlation with electrical properties, which was only recently discovered and is not well recognized. This work is concerned with the thermal depolarization process in (0.67 - x)BF-0.33BT-xSZ ferroelectrics with large strain response. Macro-to-local property characterization suggests that the largest electrostrain can be achieved in the critical component (x = 0.02) with the most flexible structure features, which is the transition point from the ferroelectric macrodomain to the relaxor nanodomain. The real-space domain image by piezoresponse force microscopy has revealed that an electric field can transform the labyrinth-like nanodomain into oriented large-size domain. Once the heating temperature is above Td, the poling-induced large-size oriented domain will transform back to its initial state. Most importantly, the thermally induced domain broken is first established with the conduction and phase transition, as disclosed by temperature dependence of DC resistivity and the pyroelectric coefficient. That is, during the thermal depolarization process, the activation energy (Ea) changes from 0.65 eV (T < Td) to 1.1 eV (T > Td), simultaneously accompanied with the structural transition from the poling-induced unstable ferroelectric state (long-range correlated rhombohedral phase) to the initial relaxor state (short-range correlated pseudo-cubic phase), which may be the driving force for the domain decays above Td. We believe that the understanding of Td in BF-BT-based relaxor ferroelectrics can provide some clues for further designing high-performance BF-BT ferroelectrics.