Phase diagram for Bi-site La-doped BiFeO3BaTiO3 lead-free piezoelectric ceramics

Three different series of lead-free ceramics, i.e., (1-y)Bi1.03(1-x)LaxFeO3–yBaTiO3 (y = 0.27, x = 0.00–0.12), (y = 0.30, x = 0.00–0.10), and (y = 0.33, x = 0.00–0.08) are prepared via a conventional solid-state reaction with water quenching. From X-ray diffraction and electrical property measurements, two morphotropic phase boundaries (MPBs) are discovered in all three ceramic systems. The first MPB (MPB-I) appeared between rhombohedral and tetragonal phases, whereas the second MPB (MPB-II) appeared between tetragonal and cubic-like phases. The highest direct piezoelectric coefficients (d33 = 201, 274, and 268 pC/N) are mainly attributed to the typical MPB-I of the rhombohedral and tetragonal phases. However, the highest converse piezoelectric coefficients (d33∗ = 490, 500, and 570 pm/V with Curie temperature > 330 °C) are obtained for compositions near to the MPB-II. A significant enhancement in the dielectric constant at low temperature is associated with the local structural heterogeneity by La3+ doping, which serves as an origin for a high piezoelectric strain response. Based on the crystal structure as well as on the dielectric, ferroelectric, and piezoelectric properties, a phase diagram is constructed for La-doped BiFeO3BaTiO3 ceramics. This phase diagram reveals the relationship between piezoelectric performance and crystal structure.