Tetragonal phase and enhanced depolarization temperature in Ba-rich (Bi, Na)TiO3-BaTiO3 lead-free piezoelectrics

Lead-free piezoelectric materials with high temperature stability attract attention on both applications and fundamental researches. In this study, oxygen vacancies are introduced by the nonequivalent chemical substitution of Ba2+ for Bi3+ in 0.94 [(Bi1xBax)(0.)Na-5(0.5)]TiO3-0.06BaTiO(3)(-delta) (x = 0, 0.1, 0.2, and 0.3) lead-free systems which are synthesized by solid state reaction method. Effects of oxygen vacancies on microstructure, grain morphology, dielectric, piezoelectric, and temperature stability are investigated. Due to the introduction of oxygen vacancies, the crystal structure changes from the morphotropic phase boundary to a tetragonal phase with a relatively large tetragonality (c/a = 1.02-1.023) which is even larger than most of the lead-free tetragonal end number. Furthermore, the temperature stability of piezoelectricity is greatly improved. With increasing Ba2+ substitution, the depolarization temperature is gradually enhanced from 108 degrees C of x = 0 (0.94BNT-0.6BT) to 200 degrees C, 213 degrees C, 230 degrees C for x = 0.1, 0.2, 0.3, respectively. The present study shows that tetragonal phase can be introduced and temperature stability can be improved in (Bi,Na)TiO3-BaTiO3 by the nonequivalent substitution of valence.