Raman Scattering Study of Piezoelectric (Na0.5K0.5)NbO₃-LiNbO₃ Ceramics

A Raman scattering study of (Na 0.5 K 0.5 )NbO 3 (NKN)-LiNbO 3 (LN) lead-free piezoceramics has been carried out on nominal compositions of (1- x )NKN- x LN (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 NbO 6 octahedra. At 0.05 ≤ x ≤0.07, the broadening of the scattering peaks corresponding to the internal modes of the NbO 6 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 v 1 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 K 3 Li 2 Nb 5 O 15 (KLN) secondary phase and an increase in the number of distorted LiNbO 3 -type crystal units.