Raman Scattering Study of Piezoelectric (Na_0.5K_0.5)NbO₃-LiNbO₃ Ceramics

A Raman scattering study of (Na_0.5K_0.5)NbO₃ (NKN)-LiNbO₃ (LN) lead-free piezoceramics has been carried out on nominal compositions of (1-x)NKN-xLN (0 ≤x ≤0.70). The Raman spectra demonstrated a variety of changes with x, mainly classified as lattice translations involving motions of the alkaline cations and internal modes of NbO₆ octahedra. At 0.05 ≤x ≤0.07, the broadening of the scattering peaks corresponding to the internal modes of the NbO₆ octahedra occurred preferably, which is consistent with the evidence for a morphotropic phase boundary (MPB). Furthermore, an abrupt shift was observed in the peak position of the symmetric stretching mode v₁ to a higher frequency, resulting from the distortion of O-Nb-O angles caused by incorporating small Li ions into the perovskite units. This is considered to be a prologue for the structural transformation in the NKN-LN solid solution from orthorhombic to tetragonal symmetry. At compositions in which x increases above the MPB, the translational mode of the Li⁺ cation emerged clearly, and the overall scattering pattern gradually changed to complex patterns caused by the formation of a tungsten-bronze K₃Li₂Nb₅O₁₅ (KLN) secondary phase and an increase in the number of distorted LiNbO₃-type crystal units.