Phase coexistence and high piezoelectric properties in (K0.40Na0.60)0.96Li0.04Nb0.80Ta0.20O3 ceramics

Lead-free (KxNa1-x)(0.96)Li0.04Nb0.80Ta0.20O3 ceramics with x = 0.10-0.70 were prepared by the conventional solid-state reaction technique. The influence of the K/Na ratio on the microstructure, crystallographic structure, phase transition and piezoelectric properties was investigated. It has been disclosed that the phase transition temperature TO-T drastically decreases with x in the narrow compositional range of x = 0.30-0.40 and the phase coexistence of the orthorhombic structure and the tetragonal structure occurs near x = 0.40. The ceramics with x = 0.40 shows high piezoelectric properties (d(33) = 254 pCN(-1), k(p) = 51.5%, k(t) = 49.4% and k(33) = 66.6%, respectively) with low dielectric loss (tan delta = 1.5%) and weak temperature dependence between 10 and 85 degrees C. In particular, the piezoelectric properties remain almost unchanged in the thermal ageing test from -125 to 300 degrees C. Therefore, this ceramic is considered to be a very promising lead-free piezoelectric material for practical applications. The relation of piezoelectric properties with morphotropic phase boundary and polymorphic phase transition was discussed.