Structure, dielectric and ferroelectric Properties of (1-x)K_0.5Na_0.5NbO₃ - xBi_0.5Na_0.5TiO₃(0 < x< 0.1) Solid Solution

In this study, we prepared lead-free (1-x)K0.5Na0.5NbO3 - xBi(0.5)Na(0.5)TiO(3) ceramics (KNNBTx) with x = 0, 0.025, 0.05, 0.075, 0.1 using the conventional solid-state reaction method. Scanning electron microscopy analysis revealed dense ceramics with a notable decrease of the average grain size from 3 mu m for x = 0-0.3 mu m for x >= 0.050. X-ray diffraction pattern refinement unveiled that the pure KNN possessed an orthorhombic Amm2 symmetry. In contrast, the KNNBT0.100 composition displayed a tetragonal P4mm symmetry, while the KNNBT0.025 composition exhibited a monoclinic Pm symmetry, suggesting the stabilization of an intermediate phase between the orthorhombic and tetragonal symmetries for x = 0.025. Dielectric measurements indicated that the introduction of small cations in A and B sites of the KNN matrix induced a diffusive phase transition. According to the modified Curie-Weiss law (named also Uchino and Nomura function), the degree of diffuseness (gamma) increased from gamma = 1.45 to gamma 1.74 with increasing x. Raman spectroscopy revealed a frequency upshift of the vibration modes along with broadening of the modes as the NBT composition increased, indicating increased structural disorder in the perovskite structure. Temperature-dependent Raman spectra exhibited the persistence of certain modes at higher temperatures due to the off-centering of cations at A and B sites, a result of the induced substitution-induced disorder in the KNN matrix. Temperature dependent of Raman modes' frequency have shown substantial modifications at temperatures close to the critical temperatures identified by dielectric measurements. Ferroelectric measurements demonstrated that the incorporation of NBT into the KNN matrix led to significant alterations in the polarization and ferroelectric characteristics of the parent ceramic (pure KNN). As the NBT content increased, the remanent polarization decreased from 13.1 to 3.6 mu C/cm(2), and the P-E curves became pinched/distorted, indicating a disruption in the long-range order of KNN due to structural and grain size modifications.