Role of structural changes in dielectric, ferroelectric properties of Sr 2K xNa 1− xNb 5O 15 lead-free ceramics

Abstract

Lead-free Sr 2K x Na 1− x Nb 5O 15 (0.00 ≤ x ≤ 0.20) piezoelectric ceramics were prepared by two-step solid state reaction method. Pure tungsten bronze structure could be obtained in all ceramics and K substitution could accelerate the phase formation at lower temperatures. The lattice constant calculation indicated expansion of the unit cell and reduced distortion of the crystal structure with K substitution due to the bigger ionic size of K + (1.64 Å) compared to that of Na + (1.39 Å). Electrical properties of Sr 2K x Na 1− x Nb 5O 15 ceramics greatly depended on the K content. Curie temperature T c shifted downward, whereas the maximum dielectric constant ɛ m and the degree of diffusion phase transition all increased initially and then decreased as K content increased, indicating that proper amount of K substitution with x between 0.05 and 0.10 could enhance the dielectric properties. All the ceramics showed an intermediate relaxor-like behavior between normal and ideal relaxor ferroelectrics according to the modified Curie–Weiss law. With increasing K content, the remnant polarization ( P r) decreased gradually and the coercive field ( E c) decreased initially and then increased. But normal ferroelectric hysteresis loops could be observed in all compositions. Besides, the underlying mechanism for variations of the electrical properties due to K substitution was explained in this work.