Irreversible phase transitions in BiFeO 3 –SrTiO 3 lead-free piezoceramics

The development of high-performance lead-free piezoelectric materials has gained significant attention due to environmental concerns regarding lead toxicity. In this study, through in situ poling synchrotron X-ray diffraction (XRD), dielectric spectroscopy, and ferroelectric measurements, we demonstrate an irreversible transition from non-ergodic relaxor behaviour to long-range ferroelectric ordering under applied electric fields in (1 − x)BiFeO3-xSrTiO3 with MnO2 addition (BF-ST-Mn). The optimal composition with x = 0.44 exhibits an electrostrain of ∼0.10% at 80 kV cm−1 through an irreversible pseudo-cubic to rhombohedral structural transformation followed by ferroelectric domain switching. This stands in sharp contrast to BF-ST-Nb systems, where strong random fields preserve a stable ergodic relaxor state; instead, the BF-ST-Mn system overcomes moderate pinning to establish irreversible long-range rhombohedral order. Synchrotron XRD reveals an initial structural transformation during the first electrical cycle, followed by domain switching in subsequent cycles. It provides promising pathways for lead-free actuator applications requiring high electrostrain at moderate driving fields.