Raman scattering in sodium nitrite crystals near the phase transition

Optical Raman spectra of a ferroelectric sodium nitrite crystal have been detected in a wide spectrum range at various temperatures, including the region of the ferroelectric phase transition. A manifestation of a transverse soft polar mode of the $A_{1}(z)$ type responsible for the ferroelectric phase transition has been discovered in the spectrum at room temperature. This mode has been found to become overdamped even far from the ferroelectric phase transition temperature. This mode also appears as a central peak under heating. It has been found that the pseudoscalar mode of the $A_2$ type has the highest intensity in the Raman spectrum of sodium nitrite. The frequency corresponding to the maximum intensity of this mode in the Raman spectrum varies from 130 cm$^{-1}$ at 123 K to 106 cm$^{-1}$ at $T$ = 513 K. A fair agreement of the experimental data for the $A_{1}(z)$ mode with the Lyddane–Sachs–Teller relation has been established. The polariton curves for the $A_{1}(z)$ polar mode and the dispersion curves for axinons has been plotted.