Low-frequency Raman spectra inLiNbO3: Within and beyond the standard paradigm of ferroelectric dynamics

Low-frequency Raman spectra of stoichiometric LiNbO3 crystals are investigated in the frequency range down to ∼1 cm−1 and in the temperature range of 300–1423 K . The central peak (quasielastic light scattering) is found and described for all temperatures studied and in both scattering geometries explored, zz - and xy -configurations. The central peak features follow the standard paradigm, predicting the critical narrowing of the width, only at T≳1300 K in the zz -configuration. At lower temperatures the inverse relaxation time increases with the temperature increase, in contradiction to the standard theoretical predictions. This experimental result was interpreted as the importance of local fluctuations of the order parameter. The ferroelectric dynamics of LiNbO3 is described by the soft mode coupled to a relaxation, sharing both displacement and order-disorder features