Effects of La-Substitution on the Ferroelectric and Piezoelectric Properties of Lead-Free BiFeO 3 -BaTiO 3 Piezoelectric Ceramics

La-substitution effects for Bi-site in 0.7Bi1.03(1-x)LaxFeO3-0.3BaTiO3 (abbreviated as BF30BT-xLF with x = 0.00 - 0.10) ceramics were investigated systematically. All these ceramics were synthesized by a conventional solid-state reaction method. From crystal structure investigation, it was found that BF30BT-0LF ceramic has a predominant rhombohedral symmetry whereas, for BF30BT-1LF composition rhombohedral-tetragonal (R-T) phases coexisted with each other as evident from the {111} and {200} peaks fitting deconvolution. However, for BF30BT-7LF ceramic, the crystal structure transforms to tetragonal and cubic-like (T-CL) mixed phase whereas above the 7 mol% cubic phase fraction become dominant. The largest direct piezoelectric coefficient (d33 = 274 pC/N) was obtained with a high Curie temperature (TC = 532 °C) for BF30BT-1LF ceramic, where the morphotropic phase boundary (MPB) was formed. Here we report that both electrical poling and cycling significantly improved the ferroelectric polarization in the hard-ferroelectric ceramics (i.e., x ≤ 0.035) and this effect gradually become weak in the soft ferroelectric (x ≥ 0.07) ceramics. Meanwhile, a high electric field induced strain (Smax = 0.27 %) with a converse piezoelectric coefficient (d33* = 500 pm/V) was obtained for BF30BT-7LF composition, which is comparable to those of Pb(Zr,Ti)O3 ceramics.