Effect of microstructure on the dielectric properties of (1−x)Na_0.5K_0.5NbO₃–xSrTiO₃ ceramics

In this paper, the dielectric properties and microstructure of lead-free ceramics with compositions of (1−x)Na_0.5K_0.5NbO₃–xSrTiO₃(NKN-ST) (x=0–0.15) are reported. The NKN-ST-based ceramics were synthesized using the conventional solid-state reaction method. When the amount of SrTiO₃ was increased, the crystal structure changed from orthorhombic to tetragonal and then pseudo-cubic symmetries, the Raman scattering spectra v₅ and v₁ modes shifted to lower wavenumbers, the grain size decreased, the dielectric relaxor behavior became stronger, and the Curie temperature shifted to a lower temperature. Furthermore, the transmission electron microscopy images and energy-dispersive spectroscopy spectra showed that the samples had a liquid phase at x≥0.08. Based on these results, it is suggested that SrTiO₃ doping in this system will cause the high temperature sintering and then cause the K-rich thin-layer liquid phase at the area around the grains which hinders the grain growth which is the response mechanism that changes the microstructure from an ordered to a disordered state. Besides, effect of microstructure on the dielectric relaxor behavior was proposed in this paper to explain the corresponding phenomenon. To the authors’ knowledge, there is no reported data provided the direct evidences to show the SrTiO₃ doping effects on the microstructure of the system and therefore the change of long-order to short-order states.