Dielectric and piezoelectric properties of (0.97-x) Bi1/2Na1/2TiO3-xBi1/2K1/2TiO3-0.03NaNbO3 ceramics

Ceramics of the series (0.97-x)Bi_1/2Na_1/2TiO₃-xBi_1/2K_1/2TiO₃-0.03NaNbO₃ (x = 0, 0.02, 0.06, 0.10, 0.16, 0.20, 0.30) were prepared by the conventional mixed oxide method. Influence of Bi_1/2K_1/2TiO₃ content on the crystal structure, microstructure, dielectric and piezoelectric properties were studied. All compositions showed single perovskite phase and the morphotropic phase boundary (MPB) between rhombohedral and tetragonal phase existed at the point of x = 0.16. Temperature dependences of permittivity and dissipation factor of unpoled samples revealed that permittivity increased with Bi_1/2K_1/2TiO₃ content and it reached maximum value near the MPB. At the same time, the peak value of dissipation factor increased with the addition of Bi_1/2K_1/2TiO₃. All the samples experienced two phase transitions: from ferroelectric to antiferroelectric at the first transition temperature (T_d) and from antiferroelectric to paraelectric at the temperature (T_m) corresponding to maximum value of permittivity. The phase transition from ferroelectric to antiferroelectric had relaxor characteristic and T_d shifted to lower temperature while increasing Bi_1/2K_1/2TiO₃ content. The best piezoelectric properties were obtained in 0.81Bi_1/2Na_1/2TiO₃-0.16Bi_1/2K_1/2TiO₃-0.03NaNbO₃ ceramic with a piezoelectric constant (d₃₃) of 146pC/N, planar electromechanical coupling factor (k_p) of 30.3% and thickness electromechanical coupling factor (k_t) of 53.2%. Abnormal piezoelectric properties were observed in the sample (x = 0.20), which was attributed to the co-existence of ferroelectric and antiferroelectric phases in it.