Role of structural modulation in electrical properties of tungsten bronze (Ca0.28Ba0.72)2.5−0.5xNaxNb5O15 ceramics

(Ca0.28Ba0.72)2.5−0.5xNaxNb5O15 ceramics (CBNN, x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0) were prepared by the conventional solid-state reaction method. Pure tungsten bronze structure could be obtained in all compositions according to X-ray diffraction (XRD) patterns. Raman spectroscopy results showed that introducing Na+ in A sites could induce stronger interaction between A-sites ions and NbO6 octahedron, and also lead to the more distortion degree of NbO6 octahedron at higher Na+ contents. It was found that accompanying with the structural evolution from 'unfilled' to 'filled' tungsten bronze type by modulating Na+ concentration, some unique dielectric and ferroelectric behavior emerged for CBNN ceramics, especially when x > 0.4. Natural ferroelectric-paraelectric phase transition at the Curie Temperature (Tc) was determined for all compositions, while only the samples with x > 0.4 showed slight dielectric anomaly around 125-200 K, which was attributed to the local structural fluctuation of stronger NbO6 octahedron distortion in the ab plane. In addition, the more intense dependence of ε on the frequency measured around Tc could be attributed to the ions disorder in mainly A2 sites caused by simultaneous occupation of Na+ and Ba2+ ions when x > 0.4. Normal ferroelectric hysteresis loops were observed in all compositions, and the related mechanisms for ferroelectric variations were discussed in detail. The better comprehensive dielectric and ferroelectric properties was obtained at x = 0.8 due to the bigger distortion degree of NbO6 octahedron polar unit and the highest densification.