Donor multiple effects on the ferroelectric and piezoelectric performance of lead-free BiFeO₃-BaTiO₃ ceramics

Nb-modified lead-free 0.67Bi_1.03FeO₃-0.33BaTi_1-xNb_xO₃ ceramics were synthesized by a solid-state reaction combined with a quenching process and investigated in terms of ferroelectric and piezoelectric properties. The Nb modification induced a phase change from the rhombohedral and tetragonal mixed-phase to the pseudocubic phase. This chemical modification transforms the long-range ferroelectric state to a short-range relaxor ferroelectric state. As a result, an excellent piezoelectric performance (d₃₃ = 285 pC/N and d₃₃* = 403 pm/V) with high a Curie temperature, 473 °C and low piezoelectric strain hysteresis, 20% were achieved. Furthermore, high d₃₃* at a very low field 25 kV/cm and the temperature-insensitive piezoelectric response suggest highly promising results in the lead-free BF-BT ceramics. It is believed that the high piezoelectric response is related to multiple effects such as crystal structure of morphotropic phase boundary, an optimum grain size (≈5 μm), high lattice distortion (c_T/a_T = 1.01) and soft-ferroelectric effect induced by the donor-doping.