Engineering Giant Strain in Bismuth Ferrite–Barium Titanate Relaxor Ferroelectrics via A-Site Driven Local Structural Disorder

Lead-free bismuth ferrite–barium titanate (BF–BT) relaxor ferroelectrics have emerged as promising candidates for high-strain actuator applications, yet the fundamental link between their nanoscale structure and macroscopic electromechanical performance remains elusive. This study overcomes this challenge by demonstrating that controlled A-site La3+ doping in 0.7(Bi0.95La0.05)FeO3–0.3BaTiO3 (BLF–BT) directly engineers a local structural environment characterized by chemical disorder and localized stress fields. Through local structure analysis and simulations, we reveal that La doping introduces A-site chemical heterogeneity and lattice mismatch, enhancing FeO6 octahedral distortions and local structural fluctuations. This pronounced local disorder suppresses long-range rhombohedral order, fostering a pseudocubic matrix populated by interacting randomly oriented polar nanoregions. These structural modifications create a flattened energy landscape that facilitates nearly isotropic and low-barrier polarization reorientation under an electric field. The resultant cooperative switching of these highly responsive nanodomains, the inherent lattice strain from local distortions, yields substantial unipolar strain of 0.35%, representing a 200% enhancement over undoped BF–BT. This work provides a definitive structural mechanism for giant strain in lead-free relaxors and establishes a design principle for activating large electromechanical responses through targeted local disorder.