Piezoelectric properties and diffusion phase transition around PPT of La-doped (Na_0.52K_0.44Li_0.04) Nb_0.8Ta_0.2O₃ lead-free piezoelectric ceramics

Lead-free ceramics (Na_0.52K_0.44Li_0.04)_1−3xLa_xNb_0.8Ta_0.2O₃ (KNLNT-Lax, x=0.00, 0.25, 0.5, 0.75, 1.00, 1.25 mol%) as non-polluting materials were prepared by solid state reaction method. The structure, piezoelectric proprieties and temperature stability of KNLNT ceramic with different La doping concentrations were investigated. The results show a transition from orthorhombic-tetragonal mix phase to tetragonal single phase with the variation of La³⁺ concentrations. The SEM micrographs of surface and fractured surface show a dense microstructure with few micropores. The La-doped KNLTN ceramic will be an alternative candidate contributes to excellent piezoelectric properties, which are found in the 0.75 mol% La-doped KNLNT ceramics, with d₃₃=215pC/N, k_p=42.8%and Q_m=89. It has been remarkably improved that the temperature stability of KNLTN-Lax piezoelectric properties at room temperature, and the dielectric relaxation can be observed obviously. The mechanism of La doping was analyzed in terms of valence compensation and polymorphic phase transition (PPT) diffusion. The orthorhombic-tetragonal phase transition around room temperature and the relaxation transition were considered contributing to the excellent piezoelectric performance and improved temperature stability of La³⁺-doped KNLTN.