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

The Raman spectra of the noncentrosymmetric biaxial sodium nitrite (NaN${\mathrm{O}}_2$) 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 $B_1\mathrm{}\left(x\right)\mathrm{}$ mode of the N$\mathrm{O}_2^{}{}_{}{}^{-}$ molecular ion were examined in some detail.