Correlation between crystal structures, Raman scattering and piezoelectric properties of lead-free Na0.5K0.5NbO3

Relationships between crystal structures, lattice parameters, bond lengths and angles, vibration modes, and dielectric and piezoelectric properties of LiNbO3, LiTaO3, and LiSbO3 substituted lead-free Na0.5K0.5NbO3 (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 BO6 octahedron. Shortening of the bond distances led to noticeable shifts toward higher frequencies in the bending vibration ν5 and stretching vibration ν1 modes. 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.