Large High-Temperature Piezoelectric Response of Lead-free BiFeO₃-BaTiO₃ Originating from Relaxor Disorder

To date, no piezoelectric material system has been developed that would match the piezoelectric performance of the group of lead-based relaxor ferroelectrics, such as Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ (PMN-PT). In the quest to find alternatives, continuous efforts have been made to understand the complex microscopic origins leading to the large macroscopic piezoelectric response of PMN-PT and similar lead-based perovskites with relaxor characteristics. Despite significant advances, it is still unclear whether those concepts can be efficiently used to develop lead-free relaxor ferroelectric alternatives. Here, a large piezoelectric response of BiFeO₃-BaTiO₃ (BFO-BTO) ceramics is reported, characterized by a d₃₃ coefficient of 1200 pm V^-1 measured at 325 °C, 1 kV cm^-1 of driving field and 90 Hz of field frequency. While composed of multiple contributions, the large response is found to be dominated by a strongly nonlinear and hysteretic process, likely associated with the dynamics of hierarchically arranged nanodomain walls, thus resembling the mechanisms operating in lead-based relaxor materials. Although the response is triggered upon elevated thermal energy, the results provide valuable information about the microscopic behavior that can be potentially used to tailor the response of lead-free relaxor ferroelectrics and bring them closer to the highly performant lead-based perovskites.