Oxygen Vacancy Engineering in Lead-Free Piezoelectric Ceramics for Performance Optimization
Lead-free piezoelectric ceramics, including potassium sodium niobate (KNN), bismuth sodium titanate (BNT), and barium titanate, are promising alternatives to lead-based counterparts due to environmental regulations. However, their functional properties—piezoelectric coefficient, mechanical quality factor, dielectric loss, Curie temperature, and thermal stability—remain inferior. Oxygen vacancy engineering has emerged as a key strategy to optimize these properties via defect modulation. Oxygen vacancies, prevalent point defects, arise from high-temperature processing, non-stoichiometry, volatile oxide evaporation, or reducing atmospheres. Acceptor doping and post-processing annealing further increase their concentration. These defects influence electrical conduction, piezoelectric/dielectric behavior, and catalytic activity. They exist as lattice vacancies, domain wall vacancies, grain boundary vacancies, and defect dipoles with cation vacancies. Their presence induces lattice distortion, hinders domain wall motion, increases coercive field, and enhances mechanical quality factor via hardening. Defect dipoles align with spontaneous polarization, creating internal bias fields that pin domains, reducing losses. Quantification of oxygen vacancies remains challenging; concentrations below 1 at% in NBT and KNN are difficult to detect. Techniques like XPS have reliability issues. A combination of impedance spectroscopy, XPS/STEM, EPR/PAS is recommended. Defect chemistry modeling, using acceptor doping to fill vacancies, allows inference of non-stoichiometry ranges. For NBT, Bi deficiency of 0.0017–0.0033 and O deficiency of 0.0025–0.0050 were calculated, corresponding to Na0.5Bi0.4967–0.4983TiO3.