Giant Piezoelectricity and High Curie Temperature in Nanostructured Alkali Niobate Lead-Free Piezoceramics through Phase Coexistence

Because of growing environmental concerns, the development of lead-free piezoelectric materials with enhanced properties has become of great interest. Here, we report a giant piezoelectric coefficient (d₃₃) of 550 pC/N and a high Curie temperature (T_C) of 237 °C in (1-x-y)K_1-wNa_wNb_1-zSb_zO_3-xBiFeO_3-yBi_0.5Na_0.5ZrO₃ (KN_wNS_z-xBF-yBNZ) ceramics by optimizing x, y, z, and w. Atomic-resolution polarization mapping by Z-contrast imaging reveals the intimate coexistence of rhombohedral (R) and tetragonal (T) phases inside nanodomains, that is, a structural origin for the R-T phase boundary in the present KNN system. Hence, the physical origin of high piezoelectric performance can be attributed to a nearly vanishing polarization anisotropy and thus low domain wall energy, facilitating easy polarization rotation between different states under an external field.