Effect of iron acceptor doping and calcium donor doping in potassium sodium niobate-based lead-free piezoceramics

Lead-free potassium sodium niobate (KNN) ferroelectrics have emerged as promising alternatives to lead-based materials due to their reduced toxicity and sometimes enhanced properties. The opportunity to modify the ferroelectric properties by doping, nevertheless, differs from lead-based ceramics. A much lower impact with increasing dopant concentration was observed. This study systematically investigates the differences in defect chemistry of Na_0.475K_0.475Li_0.065Nb_0.92Ta_0.08O₃ ceramics through iron (Fe) acceptor doping and calcium (Ca) donor doping at varying concentrations. From the defect chemical point of view, it becomes evident that the impact of intrinsic defects on the electrical properties exceeds the effect of extrinsic defects induced by doping. The performance of the donor-doped (Ca²⁺) ceramics closely resembles that of undoped KNN. Thus, hardly any change in ferroelectric properties and conductivity is observed. For the Fe-doped samples, the conductivity and its contribution to the dielectric loss increased, which is not to the benefit of an application as a ferroelectric. Therefore, the concepts of “softening” and “hardening” of ferroelectric properties by doping are incompatible with regularly sintered KNN ceramics.