Phase diagram for Bi-site La-doped BiFeO_(3)—BaTiO_(3)lead-free piezoelectric ceramics

Three different series of lead-free ceramics,i.e.,(1-y)Bi1.03(1_x)LaxFeO_(3)-yBaTiO_(3)(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(d_(33)=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(d_(33*)=490,500,and 570 pm/V with Curie temperature>330℃)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 La^(3+)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.