Structure-property relationship in lead-free A- and B-site co-doped Bi_0.5(Na_0.84K_0.16)_0.5TiO₃-SrTiO₃ incipient piezoceramics

In this work, a phase diagram of A- and B-site co-substituted 0.96[{Bi_0.5 (Na_0.84K_0.16)}_1-x-yLi_xMg_y(Ti_1-zNb_z)O₃]-0.04SrTiO₃ (abbreviated as LMN-doped BNKT-ST), where x, y and z = 0.00-0.030, was schematically constructed on the basis of crystal structure and electromechanical, dielectric and piezoelectric properties. The underlying mechanism of the compositionally-induced non-ergodic (NR) to ergodic relaxor (ER) phase transition was explored, and emphasis was given on relating the chemically-induced polymorphic structural phase transition to the dynamics of polar nano-regions (PNRs) and their random fields, which strongly affect the dielectric, ferroelectric, piezoelectric and field-induced strain properties of the investigated system. X-ray diffraction patterns revealed that LMN doping resulted in a transition from coexistence of rhombohedral and tetragonal phases to a pseudocubic phase. Both the dielectric constant and the ferroelectric-relaxor transition (T_F-R ∼ 100°C) temperature decreased with an increase in LMN content. The piezoelectric and ferroelectric responses of the BNKT-ST ceramics were significantly decreased by the addition of LMN. However, the destabilization of the piezoelectric and ferroelectric properties was accompanied by significant enhancements in the bipolar and unipolar strains. A large electric-field-induced strain (S = 0.28%) and a corresponding dynamic piezoelectric constant (S_max/E_max) of 560 pm V^-1 were observed under the driving field of 5 kV mm^-1 when 1.5 mol% LMN was substituted on respective sites. This significant strain enhancement at this composition, with LMN = 0.015, may be attributed to both the field-induced reversible structural transition and the compositionally-induced NR to ER phase transition. The composition- and temperature-dependence of the energy storage density (W) were studied to demonstrate the practicability of the LMN-doped BNKT-ST. It was found that the addition of LMN enhanced the difference between maximum polarization and remnant polarization, resulting in an improvement of the energy storage properties. For the composition with LMN = 0.020, a nearly temperature-invariant large recoverable energy density (W = 0.70 J cm^-3) was achieved under 5.5 kV mm^-1 in the wide temperature range of 100-150°C. These properties indicate that the synthesized system might be a promising lead-free candidate for actuator and energy storage capacitor applications.