Enhancing piezoelectric performance in KNN ceramics via Sb-BNZ synergistic doping

Potassium-sodium niobate (KNN)-based lead-free piezoelectrics are promising environmentally benign alternatives to lead-based ceramics, yet their performance is often limited by high polarization-reversal energy barriers. Here, a simple and effective A/B-site synergistic doping strategy using NaSbO₃ (NS) and Bi_0.5Na_0.5ZrO₃ (BNZ) is proposed to construct a stable new phase boundary (NPB) in KNN. Rietveld refinement confirms that the optimum composition (x = 0.035) exhibits rhombohedral-orthorhombic-tetragonal (R-O-T) triple-phase coexistence at room temperature. This NPB state considerably reduces the polarization-reversal barrier and is accompanied by maximized grain size (∼6.68 μm). As a result, the ceramic shows significantly enhanced electrical properties: a high piezoelectric coefficient d₃₃ = 456 pC/N, a planar electromechanical coupling coefficient k_p = 52.2%, and a greatly increased dielectric constant (ε_r ≈ 2542) with low loss. The improvement stems from the multi-orientation polarization paths and low-energy-barrier reversal enabled by the NPB, further aided by facilitated domain-wall motion due to larger grains. This work not only presents a high-performance KNN composition but also offers a straightforward doping approach to controllably design NPBs for advanced lead-free piezoelectrics.