Piezoelectric performance, phase transitions, and domain structure of 0.96(K_0.48Na_0.52)(Nb_0.96Sb_0.04)O₃−0.04(Bi_0.50Na_0.50)ZrO₃ ceramics

Demand for replacing the current lead-based piezoelectric materials with some lead-free ones becomes increasingly strong from environmental concerns. In this article, we report the piezoelectric performance, the phase transitions, and the domain configurations of highly dense 0.96(K_0.48Na_0.52)(Nb_0.96Sb_0.04)O₃−0.04(Bi_0.50Na_0.50)ZrO₃ ceramics prepared by two step-sintering through solid-state reaction. This material has outstanding piezoelectric properties of piezoelectric coefficient d₃₃ = 512 pC/N and electromechanical coupling coefficient k_p ≈ 0.56 at room temperature. While d₃₃ exhibits a broad peak and is greater than 430 pC/N between −30 °C and 70 °C, k_p depends weakly on temperature below 50 °C but decreases considerably with further increasing the temperature. In terms of thermal aging, both d₃₃ and k_p remain stable from −50 °C to 240 °C. The degradation of k_p quickly stabilizes in the first thermal cycle between −50 °C and 150 °C. Furthermore, the measurement of relative dielectric permittivity ε′ upon heating indicates that rhombohedral-orthorhombic, orthorhombic-tetragonal, and tetragonal-cubic phase transitions occur at T_R-O ≈ −40 °C, T_O-T ≈ 54 °C, and T_C ≈ 265 °C, respectively. The X-ray diffraction analysis shows that the crystalline structure at room temperature is of orthorhombic-tetragonal phase coexistence. We also investigate the domain structure with an acid etching technique. The unpoled ceramic exhibits a complicated domain pattern consisting of irregularly shaped domains of long parallel stripes separated by 180° domain boundaries from neighboring domains. In contrast, upon poling, the domain pattern becomes simpler and takes the form of long parallel stripes of diverse widths, with a hierarchical nanodomain structure appearing inside some of the broader stripes. We consider that the superior piezoelectric properties and reasonable temperature stability are closely related to the rhombohedral-orthorhombic and orthorhombic-tetragonal phase transitions and to the characteristic domain structure.