AbstractThe Raman spectra in the region 400–4000 cm−1of 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 4+, SO 42−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 re