Modulating Polymorphic Phase Boundaries and Defect Chemistry in KNN-Based Piezoceramics Through Multi-Element Doping Strategies

Lead-free potassium-sodium niobate (K0.5Na0.5NbO₃, KNN) ceramics represent a promising alternative to replace lead-based piezoelectrics in energy harvesting and actuator applications due to their high Curie temperature and favorable ferroelectric properties. This study investigates the structural, dielectric, ferroelectric, and piezoelectric properties of pure KNN and multi-element doped compositions: (K0.4Na0.48Bi0.1La0.02)(Nb0.9Ti0.1)O3 (KNN-BiLaTi) and (K0.4Na0.56La0.02Y0.02)(Nb0.9V0.05Sb0.05)O3 (KNN-LaYVSb), synthesized via conventional solid-state methods. Structural analysis (XRD/Rietveld) revealed single-phase orthorhombic perovskite structures (space group Amm2) for all samples, with no detectable secondary phases. XPS, Raman, SEM, and TEM revealed that the dopants tune defect chemistry, suppress surface alkali degradation, and refine the microstructure, especially in dense, low-porosity KNN-LaYVSb, which has pronounced lattice distortion for favorable domain wall motion. Dielectric experiments demonstrated lower orthorhombic–tetragonal transition temperatures and wider, more diffuse phase transitions. However, Curie temperatures remained high, indicating reliable operation over a wide range of temperatures. At optimal poling, KNN-LaYVSb attained the highest piezoelectric coefficient (d₃₃ = 221 pC/N) and excellent thermal stability up to 275 °C, retaining over 90% of room-temperature value and over 80% at elevated temperatures near 450 °C. The improved temperature stability arises from diffuse phase transitions, defect-dipole stabilization, and nanoscale heterogeneity due to multi-element doping. The remnant polarization increased from 3.49 µC/cm² (Pure-KNN) to 25.3 µC/cm² (KNN-LaYVSb) with significantly reduced coercive field (6.10 kV/cm). These findings illustrate that selective multi-element co-doping at A- and B-sites effectively modifies polymorphic phase boundaries near room temperature by improving defect chemistry through controlled vacancy formation and optimizing microstructure. This results in robust, high-performance lead-free piezoceramics for advanced energy harvesting and sensing applications operating across wide temperature ranges.