Thrivikraman V T, Sudheendran K. -
Lead-free potassium sodium niobate (KNN)-based piezoelectric ceramics have attracted considerable attention as environmentally friendly alternatives to conventional lead-containing piezoelectric materials. In the present investigation, the influence of A-site bismuth (Bi) substitution and B-site zirconium (Zr) and antimony (Sb) substitution on the dielectric and vibrational characteristics of KNN ceramics was systematically examined. The dielectric constant was measured over a broad frequency range to evaluate the polarization behaviour and dielectric stability of the modified ceramics. Raman spectroscopy was employed to investigate local structural distortions induced by aliovalent substitutions. The dielectric measurements reveal that all compositions exhibit a gradual reduction in dielectric constant with increasing frequency, which is characteristic of dielectric relaxation in perovskite oxides. Among the investigated samples, Bi-substituted KNN displays the highest dielectric constant throughout the measured frequency range, indicating enhanced polarization resulting from the stereo chemically active 6s² lone-pair electrons of Bi³⁺. In contrast, Zr- and Sb-substituted ceramics exhibit comparatively lower dielectric constants because of modifications in the NbO₆ octahedral framework and reduced long-range dipolar interactions. Raman spectra further confirm the structural modifications introduced by different dopants. Noticeable variations in Raman peak intensity and slight changes in vibrational modes associated with Nb–O stretching and O–Nb–O bending demonstrate lattice distortion and changes in local crystal symmetry. The enhanced Raman intensity observed for Zr-substituted KNN indicates stronger lattice distortion and improved crystallinity. The combined dielectric and Raman analyses establish a clear relationship between compositional modification, local structural ordering, and dielectric response. These findings demonstrate that selective A-site and B-site substitutions provide an effective route for tailoring the functional properties of KNN ceramics for environmentally suitable piezoelectric and electromechanical applications.