Structural modulation and electrical properties in ferroelectric niobates (Ca0.28Ba0.72)2.5−0.5xKxNb5O15 (0.0≤x≤0.6) ceramics

(Ca0.28Ba0.72)2.5−0.5xKxNb5O15 ceramics (CBKN, 0.0≤x≤0.6) with ‘unfilled’ tungsten bronze structure were prepared by the conventional solid-state reaction method. Effects of K+ concentration on the microstructure, dielectric and ferroelectric properties were investigated. It was found that modulating K+ in A sites to decrease the structural vacancy could enhance the dielectric and ferroelectric properties, which was attributed to the stronger interaction inside NbO6 octahedron and more distortion degree of NbO6 polar unit confirmed by the Raman spectroscopy results. While higher K+ concentration would deteriorate the physical and electrical properties due to the poor sintering behavior caused by the emergence of some larger anisometric pillar-type grains. Traditional temperature dependence of dielectric characteristics and well-saturated ferroelectric hysteresis loops were observed for all CBKN ceramics. The better comprehensive dielectric and ferroelectric properties were obtained at x=0.5 due to the bigger distortion degree of NbO6 polar unit and the highest densification. In addition, the frequency dependence of ε around transition temperature (Tc) and the temperature dependence of ferroelectric properties were discussed to further clarify the relationship between composition and performance.