Defect structure, ferroelectricity and piezoelectricity in Fe/Mn/Cu-doped K0.5Na0.5NbO3 lead-free piezoelectric ceramics

For perovskite Pb-based ceramics, outstanding hardening piezoelectric properties can be easily induced by acceptor dopings of Fe, Mn or Cu, but in this work, completely different hardening effects are observed in Fe/Mn/Cu-doped K0.5Na0.5NbO3 ceramics. Pure K0.5Na0.5NbO3 exhibits a well-saturated single P-E loop, giving low Qm of 72. Fe2O3-doped ceramic exhibits the combined effects of dominant donor and slight acceptor, giving a slightly slanted single P-E loop and relatively low Qm of 156. For MnO2-doped ceramic, moderate hardening properties with a slightly pinched P-E loop and relatively high Qm of 370 are exhibited. Unlike Fe2O3 and MnO2-doped ceramics, a double P-E loop and superhigh Qm of 1965 are obtained in CuO-doped ceramic. The defect structure and corresponding microscopic mechanisms in the ceramics have been systematically investigated. Our study shows that defect characteristics should be responsible for distinct hardening properties in Fe, Mn and Cu-doped K0.5Na0.5NbO3 materials.