Coexistence of Ferroelectric Phases and Phonon Dynamics in Relaxor Ferroelectric Na 0.5 Bi 0.5 TiO 3 Based Single Crystals

A combination of polarized Raman technique, infrared reflectance spectra, and first‐principles density‐functional theoretical calculations were used to investigate structure transformation and lattice vibrations of Na 0.5 Bi 0.5 TiO 3 , Na 0.5 Bi 0.5 TiO 3 –5%BaTiO 3 , and Na 0.5 Bi 0.5 TiO 3 –8%K 0.5 Bi 0.5 TiO 3 single crystals. It was found that Na 0.5 Bi 0.5 TiO 3 is of a two‐phase mixture with rhombohedral and monoclinic structures at room temperature. Correspondingly, three Raman‐active phonon modes located at 395, 790, and 868 cm −1 , which were previously assumed as A 1 modes of rhombohedral phase have been reassigned as A ′′ , A ′ , and A ′ modes of monoclinic phase in the present work. In particular, a strong low‐frequency A ′′ mode at 49 cm −1 was found and its temperature dependence was revealed. Two deviations from linearity for the abrupt frequency variation in the A ′′ mode and Ti–O bond have been detected at temperatures of ferroelectric to antiferroelectric phase transition T F–AF and dielectric maximum temperature T max . The appearance of Na–O vibrations at 150 cm −1 was found below T max , indicating the existence of nanosized Na + TiO 3 clusters. The observed Raman and infrared active modes belonging to distinct irreducible representations are in good agreement with group‐theory predictions, which suggests 9 A 1 +9 E and 36 A ′′ +24 A ′ modes for the rhombohedral and monoclinic phases of Na 0.5 Bi 0.5 TiO 3 , respectively.