Structural and piezoelectric properties of barium-modified lead-free (K_0.455Li_0.045Na_0.5)(Nb_0.9Ta_0.1)O₃ ceramics

Abstract

Ferroelectric (K_0.455Li_0.045Na_0.5)(Nb_0.9Ta_0.1)O₃ + x mol% BaCO₃ ceramic compositions with Ba²⁺ as an A-site dopant in the range of x = 0–1.2 mol% were synthesized by conventional ceramic processing route. Effect of Ba²⁺ content on the microstructure, ferroelectric, dielectric, and piezoelectric properties of the ceramics was investigated. The results of X-ray diffraction reveal that Ba²⁺ diffuse into the (K_0.455Li_0.045Na_0.5)(Nb_0.9Ta_0.1)O₃ lattices to form a solid solution with a perovskite structure having typical orthorhombic symmetry. As Ba²⁺ content increases, cell volume and tetragonality increase in the crystal structure of the ceramics. Increasing doping level of Ba²⁺ inhibits grain growth in the ceramics and reduces both the Curie temperature (T _c) and tetragonal–orthorhombic phase transition temperature (T _o-t). The bulk density, remnant polarization P _r, room-temperature dielectric constant (ε′_RT), planar electromechanical coupling factor k _p , and piezoelectric charge coefficient d ₃₃ are found to increase as Ba²⁺ concentration increases from 0 to 0.8 mol% and then decrease as Ba²⁺ content increases further from 0.8 to 1.2 mol%. High piezoelectric properties of d ₃₃ = 187 pC/N and k _p  = 48 % are found in 0.8 mol% Ba²⁺ composition. Optimum amount of Ba²⁺ dopant takes the polymorphic phase boundary region consisting of orthorhombic and tetragonal crystal structures of the ceramic system near the room temperature and enhances its piezoelectric properties.