Boosts in Polarization and Piezoelectric Responses of Lead-Free Ferroelectrics through Strain-Enhanced Glassy Coexistent Polars with High Dynamics

The miniaturization of domain size via point-defect modification has emerged as an effective strategy for optimizing piezoelectric properties in lead-free ferroelectrics, such as achieving large piezoelectric constants and slim-hysteresis electrostrain through polymorphic nanodomain design. However, this approach is not consistently reliable due to the uncontrollable deceleration of domain wall motion and polarization rotation (i.e., polar dynamics) caused by the random local fields around doping sites. In this work, an innovative strategy is proposed to enhance the polarization and electrostrain responses of nanodomain-patterned ferroelectrics through the design of a strain-enhanced glassy coexistent polar state (SGP), which can be established by incorporating large-radius cation Hf⁴⁺ into (Ba_0.84Ca_0.16)_0.985Bi_0.01(Ti_0.9Zr_0.07Sn_0.03)O₃ compositions. The strain-enhanced crossover state between the neighboring glassy tetragonal and orthorhombic domains greatly facilitates overall polar dynamics. This is evidenced by the large dielectric figure of merit ε_r/tan δ ≈ 9.9×10⁵ with thermal stability up to 23 K, surpassing most of reported lead-free ferroelectric ceramics. Importantly, compared to the initial matrix, significant improvements of 20.8% and 34.2% in maximum polarization and electrostrain amplitude are achieved, while maintaining a minimal polarization/strain hysteresis (≈5%). This work would pave a novel paradigm for designing superior functional ferroics by optimizing the domain dynamics.