Evolution of multilevel structures and electrical properties in potassium-sodium niobate-based lead-free piezoceramics by anionic fluorine engineering

Cation sites engineering has been widely used to modulate electromechanical properties in potassium-sodium niobate (KNN)-based lead-free materials. Unfortunately, anionic engineering, one of the effective strategies to regulate microstructure and optimize properties, has not been investigated sufficiently in KNN-based ceramics. Revealing the regulatory mechanism of anionic engineering from the perspective of microstructure is one of urgent aims, which can further enrich the strategies for tuning the performance in KNN-based ceramics. Here, two sets of samples [(K0.5Na0.5)0.94Li0.06NbO3, undoped ceramics; (K0.5Na0.5)0.94Li0.06NbO2.765F0.47, F-doped ceramics] are successfully synthesized to reveal the dopant-structure-property relationship for F-doped mechanism. The results of multilevel-structure ranging from micro-scale grains, to nanoscale ferroelectric domains and even to atomic-scale local structure inside nanodomains show that F is homogeneously distributed in KNN matrix with the atomic scale, indicating that F has been successfully doped into O sites. Compared with undoped samples, the microstructure is strongly affected by F substitution, such as the smaller grain size, increased TO-T, and decreased VO•• defect, and thus the evolution of electrical properties can be ascribed to the synergistic effect of its structure. Therefore, the exploration for evolution of structure and property caused by anionic engineering provides a new thought to consider how to tune the microscopic structure and thus tailor the electrical properties in KNN-based ceramics.