Dielectric and impedance spectroscopy of lead-free 0.99[(Bi_0.5 Na_0.4K_0.1)(Ti_0.980Nb_0.020)O ₃]-0.01(Ba_0.7Sr_0.3)TiO₃ ceramics

Piezoceramics 0.99[(Bi_0.5Na_0.4K_0.1) (Ti_0.980Nb_0.020)O₃]-0.01(Ba _0.7Sr_0.3)TiO₃ (BNKTN20-BST) were synthesized by a conventional solid-state reaction method, and temperature-dependent dielectric and ferroelectric studies were performed. The dielectric constant is well described by the law log(1/ε - 1/ε_m) ^γ=Clog (T - T_m) with a diffusive factor (γ) close to 1.7, which confirms that BNKTN20-BST exhibited a diffuse phase transition. Moreover, the behavior of real and imaginary permittivity versus logarithm of frequency was explained in detail in the three different ranges T<T_d, T_d<T<T_m, and T>T_m. In the range T<T_d, ε decreases almost linearly with the logarithm of frequency due to the coexistence of ferroelectric and ferroelectric relaxor mixed phase. In the ranges T_d<T<T_m and T>T_m, ε shows a more complicated dependence on frequency, which is ascribed to the presence of a non-polar phase and to a loss of interaction between polar-regions. In Cole-Cole plots, there are mainly two overlapping semicircles above 300 °C, which correspond to grains and grain boundaries in the material. The activation energies (E_a) for grains and grain boundaries were calculated from the slope of the ln σ_dc versus 1000/T and were found to be 0.46 and 0.75 eV, respectively.