Temperature dependent piezoelectric response and strain-electric-field hysteresis of rare-earth modified bismuth ferrite ceramics

The rare-earth (RE)-modified bismuth ferrite (BiFeO₃ or BFO) family of ferroelectrics have uncomplicated lead-free chemistries and simple perovskite structures. Due to the high Curie transition temperature of the parent BiFeO₃ perovskite (similar to 830 °C), they are promising piezoelectric materials for use at elevated temperatures. However, the influence of the specific RE species on the electromechanical behavior at high temperatures and above the coercive electric-field is not widely reported. Here, structural analysis over multiple length scales using X-ray diffraction, transmission electron microscopy and piezoresponse force microscopy is coupled with a high electric-field cycling study and in situ converse d₃₃ measurements up to 325 °C for three RE-BFO ceramic compositions, Bi_0.86Sm_0.14FeO₃, Bi_0.88Gd_0.12FeO₃ and Bi_0.91Dy_0.09FeO₃. The ceramics exhibit different phase assemblages with varying amounts of polar rhombohedral R3c and intermediate antipolar orthorhombic Pbam phases as a function of the RE species. During electric-field cycling at electric-fields with amplitudes of 160 kV cm^-1, peak-to-peak strains of 0.23-0.27% are reached for all three compositions. However, there are qualitative differences in the field-induced strain and electric current behavior as a function of electric-field cycling and the materials exhibit an electrical-history dependent behavior. Bi_0.91Dy_0.09FeO₃ possesses an improved d₃₃ stability as a function of temperature relative to the parent BFO perovskite and the highest depolarization temperature among the three RE-BFO compositions, with a stable d₃₃ of similar to 22 pC N^-1 up to 325 °C.