Relationships between crystal structures, lattice parameters, bond lengths and angles, vibration modes, and dielectric and piezoelectric properties of LiNbO
3, LiTaO 3, and LiSbO 3substituted lead-free Na 0.5K 0.5NbO 3(NKN) ceramics were investigated. Their crystal structures have been characterized using X-ray diffraction and analysed with the Rietveld method. The refinement results revealed that substitution with small cations to replace host ions in the NKN unit cell reduced the bond distances between B+ ions (Nb5+, Ta5+, and Sb5+) and their coordinated oxygens in the BO 6octahedron. Shortening of the bond distances led to noticeable shifts toward higher frequencies in the bending vibration ν 5and stretching vibration ν 1modes. The unit cell shrinkage and distortion, in addition to the incorporation of doped ions with higher charge density in the A and B sites, remarkably enhanced the piezoelectric and electrical properties of the doped NKN ceramics.