Fig. 2: Phonon analysis of KNbO3 and LiNbO3.

Phonon spectra of pm-3m cubic phase (a) KNbO3 and b LiNbO3. Symbols P, AP, and Tri correspond to the polar mode, anti-phase polar mode, and trigonal mode, respectively. c Schematics of the displacement directions of ions in polar (P) and anti-phase polar (AP) modes in LiNbO3. Please note that the vectors do not reflect real displacement magnitude.