Origin of superior hardening properties in KCuTa3O9-doped K0.5Na0.5NbO3 lead-free piezoelectric ceramics

K0.5Na0.5NbO3 + x mol KCuTa3O9 lead-free piezoelectric ceramics with superior hardening properties were synthesized by a solid-state reaction method. The addition of a small amount of KCuTa3O9 (x <= 0.04) induces the formation of (Cu'''(Nb) - V-O(..))' (DC1) and (V-O(..) - Cu'''(Nb) - V-O(..) )(.) (DC2) defect dipoles; however, DC2 becomes dominant in the ceramics with high levels of KCuTa3O9. The DC1 can provide a restoring force to reverse the switched polarization, inducing double hysteresis loops, while DC2 slightly impedes polarization back-switching. Because of the combination of DC1 and DC2 associates, the ceramics become gradually "hardened" with x increasing from 0 to 0.01, giving a superhigh mechanical quality factor Q(m) of 2494 at x = 0.01, and then are gradually softened with x further increasing to 0.07, exhibiting a relatively low Qm of 549 at x = 0.07. Our study shows that the control of defect structures can effectively tailor the ferroelectricity and piezoelectricity of K0.5Na0.5NbO3-based lead-free ceramics.