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compositions but also the temperatures, as shown in Figure 5a. Such a phase boundary is obviously different from that of PZT (see Figure 5b). Regardless of the presence of MPB or PPT characteristics, the polarization of a piezomaterial can be more easily rotated among different symmetries when the compositions are located at the phase boundaries, inducing an enhancement in dielectric and piezoelectric properties. However, recent advances indicate that there is a limited improvement in the d 33 values for KNN materials with O−T or R−O phase boundaries, 90−105,250−252 which is also inferior to most PZT ceramics 5 or even textured KNN. 12 As a result, the formation of R−T boundaries that are similar to PZT may become necessary to further enhance the piezoelectric activity of KNN, because the easy rotations of polarization axes can be induced in compositions near such a phase boundary. 11,35,70,76,106−117 In addition, we recently confirmed this assumption in potassium−sodium niobate piezoceramics by experimental methods, 106−116 and a larger d 33 value has been attained by constructing R−T phase boundaries. 106−112

Environment protection and human health concern is the driving force to eliminate the lead from commercial piezoelectric materials. In 2004, Saito et al. [ Saito et al., Nature , 2004 , 432 , 84 . ] developed an alkali niobate-based perovskite solid solution with a peak piezoelectric constant d33 of 416 pC/N when prepared in the textured polycrystalline form, intriguing the enthusiasm of developing high-performance lead-free piezoceramics. Although much attention has been paid on the alkali niobate-based system in the past ten years, no significant breakthrough in its d33 has yet been attained. Here, we report an alkali niobate-based lead-free piezoceramic with the largest d33 of ∼490 pC/N ever reported so far using conventional solid-state method. In addition, this material system also exhibits excellent integrated performance with d33∼390-490 pC/N and TC∼217-304 °C by optimizing the compositions. This giant d33 of the alkali niobate-based lead-free piezoceramics is ascribed to not only the construction of a new rhombohedral-tetragonal phase boundary but also enhanced dielectric and ferroelectric properties. Our finding may pave the way for "lead-free at last".

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A superior piezoelectric coefficient (d = 570 ± 10 pC N ), the highest value reported to date in potassium-sodium niobate-based ceramics, is obtained in (1-x-y)K Na Nb Sb O BaZrO - Bi K HfO ceramics. This high d value can be ascribed to the co-existence of "nano-scale strain domains" (1-2 nm) and a high density of ferroelectric domain boundaries. Therefore, ternary KNN-based ceramics demonstrate the potential for applications.

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The structural origin of enhanced piezoelectric performance and stability in KNN-based ceramics can be attributed to the hierarchical nanodomain architecture with phase coexistence.