Raman and infrared study of structural phase transitions in (NH4)3H(SeO4)2 and (ND4)3D(SeO4)2

The Raman and infrared spectra of (NH4)3H(SeO4)2 and (ND4)3D(SeO4)2 were investigated between 20 and 400 K in the 4000–10 cm−1 frequency range. Five crystalline phases were characterized spectroscopically and structural information was obtained by comparison with results previously obtained on sulphate analogues. Phase III at room temperature is strongly disordered, mainly as regards the ammonium ions, and a decrease in temperature leads to a progressive ordering; however, parent and deuterated compounds show different phase sequences. Only the ferroelectric phase V shows the spectra characteristic of a fully ordered crystal. The mechanism of phase transitions, involving motions of NH4+ and selenate entities, and the behaviour of the strong OH…︁O hydrogen bond, present in the asymmetric (SeO4HSeO4)3− dimers, are discussed.