Double hysteresis loop in Cu-doped K_0.5Na_0.5NbO₃lead-free piezoelectric ceramics

In this letter the authors report the observation of double hysteresis loops in Cu-doped K0.5 Na0.5 Nb O3 (KNN) ceramics. Unlike other ferroelectric titanates (e.g., BaTi O3), aging is not required for the ceramic to exhibit the double-loop-like characteristics. Based on the symmetry-conforming principle of point defects, it is suggested that defect dipoles are formed by the acceptor dopant ions- Cu2+ and O2- vacancies along the polarization direction after the diffuse tetragonal-orthorhombic phase transition of the ceramic. Because of the low migration rates of defects, the defect dipoles remain in the original orientation during the P-E loop measurement, providing a restoring force to reverse the switched polarization. The defect dipoles also provide "pinning" effects in the normal piezoelectric activities. As a result, the ceramic becomes "hardened," exhibiting an extraordinarily high mechanical quality factor (2500), while the other piezoelectric properties remain reasonably good: electromechanical coupling coefficients kp =39%, kt =47%, and piezoelectric coefficient d33 =82 pCN.