Microstructure, phase transition, and electrical properties of (K[sub 0.5]Na[sub 0.5])[sub 1-x]Li[sub x](Nb[sub 1-y]Ta[sub y])O₃ lead-free piezoelectric ceramics

Abstract

Lead-free ceramics (K[sub 0.5]Na[sub 0.5])[sub 1−x]Li[sub x](Nb[sub 1−y]Ta[sub y])O₃ have been prepared by an ordinary sintering technique. Our results reveal that Li[sup +] and Ta[sup 5+] diffuse into the K[sub 0.5]Na[sub 0.5]NbO₃ lattices to form a solid solution with a perovskite structure. The substitution of Li[sup +] induces an increase in the Curie temperature (T[sub C]) and a decrease in the ferroelectric tetragonal-ferroelectric orthorhombic phase transition temperature (T[sub O-T]). On the other hand, both T[sub C] and T[sub O-T] decrease after the substitution of Ta[sup 5+]. A coexistence of the orthorhombic and tetragonal phases is formed at 0.03<x<0.06 and 0.10<y<0.25 near room temperature, leading to significant enhancements of the piezoelectric properties. For the ceramic with x=0.04 and y=0.225, the piezoelectric properties become optimum, giving a piezoelectric coefficient d₃₃=208 pC/N, electromechanical coupling factors k[sub p]=48% and k[sub t]=49%, remanent polarization P[sub r]=14.2 µC/cm², coercive field E[sub c]=1.21 kV/mm, and Curie temperature T[sub C]=320 °C.