Structural dependence of piezoelectric, dielectric and ferroelectric properties of K0.5Na0.5(Nb1−2x/5Cux)O3 lead-free ceramics with high Qm

(K0.5Na0.5)(Nb1−2x/5Cux)O3 (abbreviated as KNNC, x = 0–2%) lead-free ceramics were synthetized by the solid state solution method. Pure perovskite phase with orthorhombic symmetry was observed. The evolution of the structure of KNNC was examined by X-ray diffraction (XRD), scanning electron microscopy (SEM) and Raman scattering spectra techniques. Our results revealed that, defect dipoles (Cu‴Nb–VO)′(Cu‴Nb–VO)′ were formed and provided a restoring force to reverse the switched polarization, which resulted in double P–E hysteresis loops in KNNC with Cu doping at x = 0.75% and 1%. However, non-polar defect complex (VO–Cu‴Nb–VO)(VO–Cu‴Nb–VO) caused a lattice shrinkage and the observed square shaped P–E loops in KNNC ceramics under high doping levels (x > 1%).

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► Double hysteresis loops were observed in K0.5Na0.5(Nb1−2x/5Cux)O3.
► Cu substitution caused structural discontinuity in KNNC.
► Dimeric defect complex (Cu‴Nb–VO)′(Cu‴Nb–VO)′ with a dipole moment was formed in KNNC.