Exploring the potential of polar-to-non-polar BiFeO3-SrTiO3 ferroelectric solid solution

Bismuth ferrite, BiFeO3 (BFO), has received considerable attention for high temperature piezoelectric applications due to its high Curie temperature (Tc) of ~830 °C. In addition, BFO based solid solutions are of great... [ view full abstract ]

Bismuth ferrite, BiFeO₃ (BFO), has received considerable attention for high temperature piezoelectric applications due to its high Curie temperature (T_c) of ~830 °C. In addition, BFO based solid solutions are of great interest due to the possibility to exhibit a morphotropic phase boundary (MPB), where the electromechanical properties are enhanced. Lately, much research has been done on conventional polar-to-polar MPBs (e.g., BFO-BaTiO₃, BFO-PbTiO₃), where the piezoelectric properties are considered to be enhanced through the polarization rotation mechanism, a mechanism commonly used to explain MPB-like property enhancements in lead-based perovskites.

Here, we investigated the solid solution between BFO and SrTiO₃ (ST) (BFO-ST) due to the possibility to exhibit polar-to-non-polar MPB, which is often ignored in the literature and where the piezoelectric properties may be enhanced through polarization extension mechanism. The system was studied from the aspect of processing, structural, microstructural and functional properties.

We found that the conventional solid state synthesis results in formation of bismuth rich phase on grain boundaries and grain junctions. However, the processing issues were eliminated with use of mechanochemical activation assisted synthesis, which was confirmed by field-emission scanning electron microscopy and high-resolution transmission electron microscopy. Measurements of the dielectric permittivity at elevated temperature showed the presence of two peaks, i.e., frequency dependent, characteristic for relaxor-like behavior and frequency independent, which is most probably associated with a ferroelectric-to-paraelectric phase transition. These results suggest that the BFO-ST ceramics consist of both a ferroelectric and a relaxor phase. We observed typical ferroelectric hysteresis loops in the whole (x)BFO-(1-x)ST compositional series with high remanent polarization (30-50 μC/cm²) and a maximum d₃₃ value of 69 pC/N, which is 75% higher than that determined for unmodified BFO (~40 pC/N). Finally, the domain switching under the applied electric field was studied using in situ synchrotron X-ray diffraction.