Lead-free BiFeO₃-BaTiO₃ ceramics attract widespread attention over the last two decades due to their high Curie temperature (T_C) and excellent piezoelectric performance. Here, in the Nd-modified 0.67BiFeO₃-0.33BaTiO₃ ceramics, an excellent piezoelectric constant (d₃₃) of 325 pC/N was achieved by applying a novel poling method (AC-bias + DC-bias) with a high T_C of 455 ℃. In addition, an ultrahigh normalized piezoelectric strain (d₃₃* = S_max/E_max) of 808 pm/V was obtained at the normal/typical and relaxor-ferroelectrics phase boundary simultaneously with good thermal stability (Δd₃₃*(T) ≈ 20%) in the temperature range of 25–125 ℃. The piezoelectric force microscopy results show the domain miniaturization from micro to nanoscale/polar nano-regions due to local structure heterogeneity caused by Nd doping. The mechanism for the giant piezoelectric strain is attributed to the thermal quenching, nano-domains, and reverse switching of the short-range order to the long-range order under the applied electric field. The strategic design of domain engineering and a proposed model for the high piezoelectricity is successfully supported by the phenomenological relation and Gibbs free energy profile. In this work, a new lead-free single-element modified BiFeO₃-BaTiO₃ ceramics was developed by applying a synergistic approach of domain engineering and phase boundary for the high-temperature piezoelectric performance.

Core–shell structured Bi_0.5Na_0.5TiO₃KTaO₃ + x% (in mass) Li₂CO₃ ceramics were fabricated in this study. Increasing x from 0 to 2 leads to the decrease of sintering temperature from 1 175 ℃ to 1 020 ℃. The limited diffusion of Ta⁵⁺ results in chemical heterogeneities and core–shell microstructures. The Ta⁵⁺-depleted cores show the nanodomains (~10 nm), while the Ta⁵⁺-rich shells display the polar nanoregions (1–2 nm). From x = 0 to 1, the appearance of cores with nanodomains contributes to the increase of dielectric constant and maximum polarization, while the further addition of Li₂CO₃ suppresses the dielectric and polarization responses due to the reduced grain sizes and polarization coupling. The enhanced dielectric relaxation and existence of core-shell microstructure with different polarization levels help to optimize the dielectric temperature stability. The x = 2 ceramics exhibit a stable high dielectric constant ~1 400 over a wide temperature range of 20–520 ℃. More encouragingly, the ultrafine grain size and core–shell microstructure in the x = 2 ceramics greatly benefit the improvement of breakdown strength. Combined with the delayed polarization saturation and high ergodicity, a high recoverable energy density of ~5.07 J/cm³ is obtained under 44 kV/mm, with a high efficiency of ~85.17%.