Phase evolution and origin of the high piezoelectric properties in lead-free BiFeO3-BaTiO3 ceramics

La-substitution effects for Be3+-site in 0.7Bi(1.)(03(1)(-x)())La(x)FeO(3)-0.3BaTiO(3) (abbreviated as BF3OBT-100xLF with x = 0.00, 0.01, 0.035, 0.06, 0.07 and 0.10) ceramics were investigated systematically. All ceramics were synthesized by a conventional solid-state reaction method and quenched in water from its sintering temperature. The crystal structure Rietveld refinement shows that undoped BF3OBT ceramic exhibited dominant rhombohedral (R) symmetry and gradually changed to the tetragonal (T) phase with La-doping. However, for x >= 0.07 compositions the lattice distortions (c(T)/a(T) and 90 degrees - alpha(R)) significantly decrease as a result crystal structures become close to the cubic-like (CL) phase. Hence, two different morphotropic phase boundaries (MPBs) were reported for BF3OBT-100xLa ceramic system; one MPB-I between the R and T phases and the other MPB-II between T and CL phases. The largest direct piezoelectric coefficient (d(33) = 274 pC/N) with a high Curie temperature (T-c = 532 degrees C) for BF3OBT-1LF composition was obtained due to the typical MPB-I between R and T phases. However, a maximum electric field-induced strain (S-max = 0.27%) with a high converse piezoelectric coefficient (d(33)* = 500 pm/V) for BF3OBT-7LF ceramic was mainly attributed to the MPB-II of T + CL phases and soft ferroelectric switching properties.