Enhanced strain of BiFeO₃-BaTiO₃ relaxor ferroelectrics ceramics: domain structure evolution induced by electric-fields and temperature

The study of BiFeO₃-0.3BaTiO₃ ceramics has gained significant attention due to their high Curie temperature (T_C ≥ 450 °C) and excellent piezoelectric properties (d₃₃ ≥ 200 pC·N⁻¹). These are particularly pronounced near the morphotropic phase boundary (MPB) region where coexisting rhombohedral and pseudocubic (R-PC) phases are observed. In addition, as the BaTiO₃ content increases, BiFeO₃-BaTiO₃ ceramics gradually become dominated by a single pseudocubic (PC-) phase. This shift results in a decrease in piezoelectric properties but an enhancement in strain performance. However, the underlying mechanism remains unclear. The high strain properties observed in non-MPB compositions provide a motivation for further investigation into these mechanisms. This paper presents a detailed analysis of the electric-field and temperature-induced domain structure evolution in BiFeO₃-0.4BaTiO₃, which is predominately characterized by the PC phase. Piezoresponse force microscope (PFM) observations reveal the presence of nanodomains and stripy domains associated with polar nanoregions (PNRs), as well as relaxor ferroelectrics (RFEs) and/or ferroelectrics (FEs). The RFEs exhibit a significantly better strain response than the FEs, providing direct evidence for the enhanced strain properties of RFEs. Elevated-temperature Raman spectroscopy confirms a decrease in B-O bonding and BO₆ deformation, along with an increase in structural symmetry, indicating the formation of RFEs and/or PNRs. The phase diagram shows the Burns temperature (T_B), dielectric maxima temperature (T_m) and freezing temperature (T_f) evaluated from the dielectric spectra; the temperature-induced evolution of domain structures; and the sequential quasi-dielectric states: PNRs, RFEs and FEs. The evolution of the domain structure, including the morphology and ratio of FEs, RFEs and PNRs, induced by either electric-fields or temperature strongly affects the strain properties of RFEs. A superior piezoelectric coefficient of d₃₃^* = 533 pm·V⁻¹ at 40 kV·cm⁻¹ and a large electric strain of S_uni = 0.285% are obtained. These results further validate that domain modulation can effectively enhance the strain properties of BiFeO₃-BaTiO₃ ceramics, which makes them promising candidates for actuator applications.