Dielectric properties of (Na0.5Bi0.5)1 − xMexTiO3 ceramics near morphotropic phase boundary

Abstract

Ceramic samples of modified ferroelectric sodium-bismuth titanate (Na_0.5Bi_0.5)_0.87 Me_0.13TiO₃ (Me = Pb, Sr and Pb + Sr), were prepared using conventional solid state reaction techniques. The studies of powder X-ray diffraction of the obtained compounds revealed their rhombohedral symmetry at room temperature, the increase of lattice constant and the increase of rhombohedral lattice distortion (except for the material with Sr dopand, where distortion decreases). Temperature (at room temperature to 400°C) and frequency (at 20 Hz to 1 MHz) dielectric measurements reveal that A-site cations addition of Pb and/or Sr have resulted in the increase of relative electric permittivity. However, the temperature T _m (when the electric permittivity is a maximum) increases after Pb or Sr doping and it decreases after (Pb + Sr) doping. The pyroelectric and current loop measurements have shown that all samples were ferroelectric. The results of these measurements also allowed us to determine the temperature variation of the remanent and spontanous polarizations. The polarizations are found to decrease after Pb or Sr doping and increase after (Pb + Sr) doping. The piezoelectric constants (d ₃₃ and d ₃₁) and electromechanical coupling factors (k ₃₃ and k ₃₁) were obtained from resonance-antiresonance measurements method. The best piezoelectric and electromechanical properties have NBT doped by Pb. This ceramic may be good candidate for device applications. The diffuse ferroelectric phase transition of the investigated materials, similarly as for pure NBT, has been revealed. The properties of these materials (especially in diffuse phase transition range) can be explained by the behaviour of polar regions.