A Raman spectroscopic study of a single crystal of sodium ammonium selenate dihydrate, NaNH4SeO4·2H2O over the temperature range 293–89 K

Abstract The Raman spectra of single crystals of NaNH 4 SeO 4 ·2H 2 O in various orientations are reported for the temperature range 293–89 K. NaNH 4 SeO 4 ·2H 2 O undergoes a transition at 180 K from a paraelectric phase to a ferroelectric phase. The changes in the Raman spectra as the crystal is cooled support the evidence from magnetic resonance studies that the transition is of a displacive type with an associated overdamped soft mode. Ordering of the NH ions but not of the complete crystal occurs at the transition temperature.