(1 - X)(K_0.48Na_0.52)(Nb_0.95-y-zTa_zSb_y)O₃-xBi_0.5(Na_0.82K_0.18)_0.5ZrO₃ lead-free ceramics: Composition dependence of the phase boundaries and electrical properties

In this work, (1 - x)(K_0.48Na_0.52)(Nb_0.95-y-zTa_zSb_y)O₃-xBi_0.5(Na_0.82K_0.18)_0.5ZrO₃, {abbreviation: KNNST-BNKZ-x-y-z} lead-free piezoceramics were prepared by a conventional solid-state reaction method, and the composition dependence of their phase structures and electrical properties was systematically discussed. Doping with Sb⁵⁺, Ta⁵⁺, and BNKZ plays an important role on the phase boundaries as well as piezoelectric activity. A three-phase coexistence involving rhombohedral-orthorhombic-tetragonal (R-O-T) phases was observed in the ceramics with 0.0325 ≤ x ≤ 0.05, 0.035 ≤ x ≤ 0.065, 0.05 ≤ z ≤ 0.08, indicating that doping with BNKZ, Ta⁵⁺, and Sb⁵⁺ can induce the formation of such a phase boundary by simultaneously increasing T_R-O and decreasing T_O-T. Enhanced piezoelectric behavior was observed in the ceramics located in the composition region of the R-O-T phase boundary, and a high d₃₃ value of 400 pC N^-1 can be attained by refining their compositions (e.g., x = 0.0325, y = 0.035, and z = 0.05), together with a high T_C value of ∼240 °C. Of particular interest is that a large electric field-induced strain of 0.18% (S_max/E_max = 706 pm V^-1) was also found in the ceramics with x = 0.0325, y = 0.035, and z = 0.05 under a low electric field of 2.5 kV mm^-1. As a result, the piezoelectric activity as well as the strain can be operated in the material system by refining x, y, and z content.