Phase coexistence induced strong piezoelectricity in K_0.5Na_0.5NbO₃-based lead-free ceramics

For perovskite ceramics, the ferroelectric phase boundary plays an important role in improving the piezoelectricity of the materials. In this work, (1 - x)(K0.5Na0.5)NbO3 - x[NaSbO3 + Bi0.5(Na0.8K0.2)0.5(Zr0.5Hf0.5)O3] lead-free ceramics with R-O-T ferroelectric phase coexistence were developed and the relationship between the phase structure and piezoelectricity was investigated in detail. As x increases, the transition temperature of the rhombohedral and orthorhombic phases (TR-O) increases, while the orthorhombic-tetragonal phase transition temperature (TO-T) decreases. When 0.04 ≤ x ≤ 0.05, three ferroelectric phases (R-O-T) coexist near room temperature in the ceramics. Due to the highly consistent orientation of the ferroelectric dipole and the free energy flattening and a low energy barrier induced by the coexistence of three ferroelectric phases (R-O-T), excellent piezoelectric performances of d33 = 452 pC N-1, kp = 63% and εr = 4414 are achieved at x = 0.04. Our study suggests that compared with two ferroelectric phase boundaries (R-O and O-T), the coexistence of the three ferroelectric phases (R-O-T) can effectively enhance the piezoelectric properties of (K0.5Na0.5)NbO3-based ceramics.