Influence of defect chemistry on ferroelectric properties of KNN

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The lead-free ferroeletric ceramic (K0.5Na0.5)NbO3 (KNN) stands out because of its high piezoelectric coefficient d_33, excellent strain properties, high Curie Temperature T_C and mechanical quality factor Q_m. Despite the optimized and promising properties, KNN is rarely found in applications so far. This can be mainly attributed to the comparably high leakage currents and low density. The high leakage currents often result from extrinsic and intrinsic point defects. It was assumed that the knowledge from point defect thermodynamics on lead-based ceramics can be directly transferred to the lead-free material. This is, however, not the case. For example, in case of (Na1/2Bi1/2)TiO3 extremely high oxygen ionic conductivity instead of ferroelectric hardening was found for acceptor doping by Li et al. For KNN, doping strategies usually have a far lower impact on ferroelectric properties as it could be expected from the experience with lead-containing material. Interestingly, defect chemical investigations have recently also been conducted on lead-based ceramics to elucidate aging and degradation mechanisms. Thus it is of high importance to determine the defect chemical properties of lead-free ceramics right from the beginning to rationalize whether high reliability criteria can be fulfilled. A combination of experimental investigations based on conductivity experiments and quantum mechanical modelling will lead to a description of the influence of different dopands on KNN. Furthermore, the effect of microstructure and phase transitions will be taken into account to separate these from changes in defect chemistry. The ferroelectric properties will be determined as a function of height and application duration of electrical field. This way possible degradation and aging phenomena due to migrating defect species can be elucidated.