Role of structural changes in dielectric, ferroelectric properties of Sr2KxNa1−xNb5O15 lead-free ceramics

Lead-free Sr2KxNa1−xNb5O15 (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 Sr2KxNa1−xNb5O15 ceramics greatly depended on the K content. Curie temperature Tc shifted downward, whereas the maximum dielectric constant ɛm 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 (Pr) decreased gradually and the coercive field (Ec) 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.