A study of the internal frequency region (400-4000 cm−1) of the Raman spectrum of a single crystal of sodium ammonium sulphate dihydrate, NaNH4SO4·2H2O over the temperature range 293-87K

The Raman spectra in the region 400–4000 cm−1 of single crystals of NaNH4SO4·2H2O in various orientations are reported for the temperature range 293–87K. NaNH4SO4·2H2O undergoes a transition at 101 K from a paraelectric phase to a ferroelectric phase. The changes in the Raman spectra of the constituent species, NH, SO and H2O as the temperature is lowered indicate that below the phase transition there are non-equivalent ions in the unit cell. The abruptness of the spectral changes at 101 K suggests a large lattice distortion in accord with the predictions of magnetic resonance studies.