Defect-engineered internal bias field enables highly efficient strain enhancement in lead-free BF-BT ferroelectric ceramics

Defect engineering has been widely explored as an effective route to modulate the electromechanical response of piezoelectric ceramics. However, achieving a high strain gain per defect design is often constrained by the competition between bias enhancement and defect-induced pinning. Here, we systematically investigate the defect-mediated electromechanical behavior of Mn-doped BiFeO 3 –BaTiO 3 -based ceramics with controlled defect concentrations. It is demonstrated that introducing an appropriate level of B-site aliovalent Mn dopants effectively amplifies the internal bias field while preserving ferroelectric switch ability leading to a pronounced bias-assisted strain amplification. In the optimal composition ( x = 0.005), the strain increases from 0.02% to 0.23% at 3 kV/mm and from 0.06% to 0.36% at 4 kV/mm, corresponding to ∼1050% and ∼500% enhancements, respectively and a high large-signal piezoelectric coefficient d 33 * ≈ 900 pm/V. Structural and electrical analyses reveal that low Mn doping promotes the formation of dense nanodomains and facilitates the aging-induced ordering of defect dipoles, whereas excessive Mn incorporation induces strong lattice disorder and defect pinning, suppressing bias-field formation and strain response. These findings establish an effective defect–structure–bias-field design principle for enhancing electromechanical strain and strain-amplification efficiency in lead-free ferroelectric ceramics. • Novelty of this work: Combining defect engineering with internal bias-field modulation enhances electromechanical strain in BF-BT ceramics; • Defect-chemistry reveals excessive doping degrades strain via altered compensation, lattice disorder and pinning that suppress bias formation; • A physical model links defect concentration to bias evolution, explaining optimal defect window for strain enhancement.