Fe3+-doped (Pb0.94Sr0.05La0.01)(Zr0.53Ti0.47)O3 (PSL(ZT)1-x-Fex) piezoelectric ceramics were prepared by the solid-state reaction method and with a variation of the Fe3+ content. When the Fe3+ content was less than 0.010, the ceramics exhibited the features of soft piezoelectric ceramics with a large remnant polarization (Pr) of 35.7 μC/cm2, a large bipolar strain of 0.22% and a high piezoelectric coefficient (d33) of 412 pC/N. The number of oxygen vacancies increased and the domain walls were pinned by the defect diploes with a further increase of the Fe3+ content. Meanwhile, the PSL(ZT)1-x-Fex ceramics showed typical hard behavior and the mechanical quality factor Qm was as high as 500. The softening-hardening transition of electrical properties was also systematically analyzed by adjusting the oxygen vacancies, the space charges and the difference between the unipolar strain and the value of d33×E.

•Li-doped Ba0.85Ca0.15Ti0.9Zr0.1O3 (BCZT) lead-free piezoceramics were prepared by the two-step synthesis and solid-state reaction method.•Their sintering temperature decreases from about 1540 °C down to about 1400 °C.•With the proper addition of Li, the densities and grain sizes of ceramics increase.•The ceramics not only have the characteristics of hard piezoceramics but also possesses the features of soft piezoceramics at low sintering temperature.Li-doped Ba0.85Ca0.15Ti0.9Zr0.1O3 (BCZT) lead-free piezoelectric ceramics were prepared by the two-step synthesis and the solid-state reaction method. The density and grain size of ceramics sufficiently increases by Li-doped sintering aid, and their sintering temperature decreases from about 1540 °C down to about 1400 °C. X-ray diffraction reveals that the phase structure of Li-doped BCTZ ceramics is changed with the sintering temperature, which is consistent with their phase transition observed by the temperature-dependent dielectric curves. The well-poled Li-doped BCZT ceramics show a high piezoelectric constant d33 (512 pC/N) and a planar electromechanical coupling factor kp (0.49), which have the characteristics of soft Pb(Zr,Ti)O3 (PZT) piezoceramic, on the other hand, the mechanical quality factor Qm is about 190, which possesses the features of hard PZT piezoceramics. The enhanced properties of the Li-doped BCZT are explained by the combination of Li-doped effect and sintering effect on the microstructure and the phase transition around room temperature.

(1 − x)K0.02Na0.98NbO3–xBaTiO3 ceramics were prepared by the solid state reaction method, and their electrical properties were investigated. The samples showed crystal structure changing from monoclinic to orthorhombic, and then to tetragonal, with an increase in BaTiO3 content. The addition of BaTiO3 markedly enhanced ferroelectric and piezoelectric properties of K0.02Na0.98NbO3 ceramics. Remnant polarization increased and coercive field decreased only in the samples with small amount of BaTiO3. Piezoelectric properties were improved with the addition of BaTiO3. The 0.9K0.02Na0.98NbO3–0.1BaTiO3 ceramics showed maximum piezoelectric constant (d33 = 160 pC/N), which was even comparable with that of (1 − x)K0.5Na0.5NbO3–xBaTiO3 ceramics. Their good piezoelectric properties, along with a low ferroelectric–ferroelectric transition temperature (TF–F), made the 0.9K0.02Na0.98NbO3–0.1BaTiO3 ceramics a potential candidate for lead-free piezoelectric applications.Highlights► Lead-free piezoelectric ceramics (1 − x)K0.02Na0.98NbO3–xBaTiO3 with good piezoelectric properties were obtained. ► The crystal structures of (1 − x)K0.02Na0.98NbO3–xBaTiO3 ceramics change from monoclinic to orthorhombic, and then to tetragonal, with an increase in BaTiO3 content. ► The 0.9K0.02Na0.98NbO3–0.1BaTiO3 ceramics have high d33 (160 pC/N) with low ferroelectric-ferroelectric transition temperature (−15 °C).

The domain structure of lead-free ceramics [(Na0.7K0.2Li0.1)0.5Bi0.5]TiO3 was studied by piezoresponse force microscopy (PFM) method. The complicated curved domain structure was observed in the ceramics, and there are some nano domains in the sub-microsized domains, which indicate the relaxor nature of the material. The mechanism for the strong relaxation of the material was discussed in the letter. The reversal behavior of the domain was also studied by PFM method. Only part of the domains reversed after the poling process, and domains of the ceramics reversed back from the center of the domains at first.