Synthesis-Phase–Composition Relationship and High Electric-Field-Induced Electromechanical Behavior of Samarium-Modified BiFeO₃ Ceramics

Solid-state (non-activated) and mechanochemical activation (activated) synthesis methods were used to produce Sm-modified BiFeO₃ ceramics of composition Bi_0.88Sm_0.12FeO₃. The first part shows that the formation of Bi_0.88Sm_0.12FeO₃ using the two synthesis methods followed a different reaction pathway on annealing the powders. The non-activated ceramics reacted by forming two intermediate phases, isostructural to BiFeO₃ and SmFeO₃, and then inter-diffusing, forming the final Bi_0.88Sm_0.12FeO₃ solid solution. Unlike the non-activated samples, the activated ceramic powders formed Bi_0.88Sm_0.12FeO₃ phase on annealing the powders, without apparent intermediate phases. As revealed by transmission electron microscopy, the non-activated reaction pathway caused the Pbam phase to form as chemical inhomogeneous (Sm-rich) isolated nano-sized grain inclusions in the final ceramics. Conversely, the activated reaction pathway caused the Pbam phase to form chemically homogeneous nano-regions within the R3c phase grains. The results demonstrate the important role of processing in the appearance of the frequently discussed anti-polar Pbam phase in this system. In the second part, the high electric-field-induced polarization and strain behaviors of these ceramics were studied by means of polarization–electric (P–E) and strain–electric field (S–E) hysteresis loops, and the S–E loops were compared with those of unmodified BiFeO₃. Bipolar S–E loops of Bi_0.88Sm_0.12FeO₃ had a distinctive butterfly shape with less frequency dependence relative to BiFeO₃ at driving-field frequencies of 0.1–100 Hz. BiFeO₃ ceramics exhibite strong driving electric-field-frequency-dependent domain switching, the origins of which were previously attributed to a domain-wall pinning mechanism and “hardening” behavior. This study shows that Sm-modification induces a “hardening–softening” transition in BiFeO₃ ceramics.