Analysis of the Temperature-Dependent Phonon Structure in Sodium Nitrite by Raman Spectroscopy

Abstract

The temperature behavior of the TO-phonon frequencies and line shapes in NaN${\mathrm{O}}_2$, as observed by Raman spectroscopy, is reported for temperatures from 20 to 250°C. The Raman spectra of the ferroelectric phase obeyed the selection rules of the $C_{2V}^{20}$ space group. In the paraelectric Raman spectra, extra resonances (six instead of the expected three) were found. This anomaly and accompanying line asymmetries are accounted for by recognizing the effect of the nitrite-ion disordering along the $b$ axis upon lattice states. Additionally, in the paraelectric Raman spectra [polarized and ($\mathrm{bc}$)], a redundant appearance of the symmetric internal vibrations was observed, from which we conclude that above the phase-transition temperature there is a large torsional oscillation of the N${\mathrm{O}}_2^{-}$ ion about the $a$ axis. Significantly, the librational frequency associated with this torsional oscillation softened at key temperatures, the temperatures at which long- and short-range orders are lost. From our data, we conclude that the nitrogen atom jumps between two equivalent lattice sites by the flipping of the nitrite ion about the $a$ axis, and we suggest that temperature-dependent coupling between normal modes precipitates this flipping. A careful search of the Raman spectra 3 ${\mathrm{cm}}^{-1\mathrm{}}$ from the frequency of the incident laser revealed no "Cochran" soft modes or supernumerary modes, such as a dipole wave, that might explain the ferroelectric phase transition in NaN${\mathrm{O}}_2$.