Unleashing pyroelectricity enhancement via phase transition-driven defect alignment in KNN-based ferroelectrics

The pyroelectric effect plays a critical role in thermal imaging and energy harvesting. Despite extensive efforts to enhance performance through doping and composite engineering, the mechanisms underlying defect dipole coupling with phase structures remain poorly understood, impeding the advancement of defect-engineered symmetry modulation. Here, we report an abnormal pyroelectric phenomenon where the pyroelectric coefficient (p) increases notably when poling temperature exceeds the orthorhombic-to-tetragonal phase transition temperature (T_O-T) in potassium sodium niobate ceramics. The p at 200°C (p = 45.4 × 10^-4 C m^-2 K^-1) rises more than sevenfold compared to poling within the orthorhombic phase (p = 6.5 × 10^-4 C m^-2 K^-1), representing the highest value reported to date and offering benefit for high-temperature thermal sensing. A dual mechanism is proposed, involving rigid-ion displacement and defect dipole alignment, which respectively contribute to increased displacement charge and space charge. Our findings establish a paradigm for optimizing high-temperature pyroelectrics through a simple, symmetry-confined thermal poling.