Specific features of the central peak in the Raman spectra of a lithium niobate crystal

The low-frequency Raman scattering (RS) spectra of a LiNbO3 ferroelectric crystal are studied in the temperature range 300-1423 K. The central peak characterizing the relaxation susceptibility of the crystal lattice is observed over the entire temperature range studied, including at temperatures much lower than the Curie temperature (T-c = 1470 K). Far from T-c, the characteristics of the central peak are shown to be unlike those expected in the framework of the standard approaches. (i) The central-peak width gamma(R) increases as the temperature increases to T similar to 1300 K, and the critical slowing down (gamma(R) proportional to T-c-T) occurs only above this temperature. (ii) A central peak arises in the RS geometry where the scattering by "nonferroelectric" E phonons is allowed. The experimental results are interpreted with allowance for relaxation dynamics in local regions of the crystal lattice.