Electromechanical and electrical properties of Bi_0.5Na _0.5Ti_1-xMn_xO _3-δ ceramics with high remnant polarization

The perovskite oxides Bi_0.5Na_0.5Ti_{1- x}Mn_xO_3-δ (BNTM10000x, x = 0%, 0.25%, 0.5%, 0.75%, 1%, 1.5%, 2%) were prepared via the conventional solid state reaction method. The crystal structure of BNT and BNTM25 ceramics were refined by the Rietveld method with powder X-ray diffraction at room temperature. A production of small amount of oxygen vacancies, a significant enhancement in tetragonality and a grain size effect due to the MnO₂ doping, led to the enhancements of the electromechanical and electrical properties of Bi _0.5Na_0.5Ti_1-xMn_xO _3-δ ceramics, and the optimized modification was x = 0.25%. The electrical properties of modified samples were deteriorated with further doping (x = 0.5-2%), owing to an obvious ferroelectric to relaxor phase transition and an increase in the concentration of oxygen vacancies. The BNTM25 ceramics was found to have a high remnant polarization (P_r) of 48.5 μC/cm², an electromechanical coupling factor (k_p) of 0.18, an electro-field-induced strain (S_total) of 0.24% and a piezoelectric coefficient (d₃₃) of 105 pC/N. In addition, almost no profound fatigue was observed after switching over 10⁶ cycles at room temperature, indicating a potential application as lead-free piezoelectric materials.