• • Oxygen vacancy concentrations below 1 at% in NBT and KNN ceramics are not reliably quantifiable by any single technique; XPS has known reliability issues, necessitating a multi-method approach (impedance spectroscopy, XPS/STEM, EPR/PAS) for accurate assessment.
• • Defect chemistry modeling via Nb5+ doping in NBT revealed Bi deficiency ranges of 0.0017–0.0033 and O deficiency of 0.0025–0.0050, corresponding to a chemical formula of Na0.5Bi0.4967–0.4983TiO3, enabling precise stoichiometric control.
• • Activation energy (Ea) shifts from <0.9 eV (oxide ion conduction) to 1.5–1.8 eV (intrinsic electronic conduction) upon filling oxygen vacancies, providing a quantitative marker for vacancy concentration changes.
• • Oxygen vacancy-induced hardening increases mechanical quality factor and stabilizes electromechanical response under high-power conditions, but also raises coercive field and suppresses polarization, requiring trade-off optimization for specific applications.