Structural and functional characterisation of KNNS-BNKZ lead-free piezoceramics

Promising piezoelectric properties have been reported recently for lead-free 0.96(K0.48Na0.52Nb0.95Sb0.05)-0.04Bi(0.5)(Na0.82K0.18)(0.5)ZrO3 (KNNS-BNKZ) ceramics. The presence of coexisting rhombohedral and tetragonal phases is thought to play a key role in their functional properties, but a thorough understanding is currently lacking. In this experiment, (1-x)KNNS-(x)BNKZ ceramics with x = 0-0.05 were prepared by the mixed-oxide method. High-resolution synchrotron X-ray powder diffraction (SXPD) measurements reveal that the addition of BNKZ into KNNS ceramics leads to an increase in the rhombohedral-orthorhombic phase transition temperature (TR-O) and a decrease in the orthorhombic-tetragonal phase transition temperature (TO-T) leading to orthorhombic-tetragonal and rhombohedral-tetragonal phase coexistence at room temperature for compositions with x = 0.02 and 0.04, respectively. By combining the SXPD results with microstructure, evidence is also found for the occurrence of chemical heterogeneity, which could provide an additional means to control the functional properties. The structural observations are correlated with changes in the dielectric and ferroelectric properties.