Raman Spectra and Mode Frequency Shifts of Ferroelectric Sodium Nitrite at 77 and 294°K

The Raman spectra of the noncentrosymmetric biaxial sodium nitrite (NaNO₂) single crystal at 77 and 294 ^∘{}K are presented. No major difference between the two spectra is observed, indicating that the recently reported phase transition at 178 ^∘{}K from thermal-expansion measurements does not appear to change the crystal symmetry. Frequency shifts were observed with changes in the phonon wave-vector orientation for all five asymmetric B modes. The frequency shifts of the asymmetric internal vibrational B₁(x) mode of the NO₂^- molecular ion were examined in some detail.