Vibrational study of the paraelectric superionic phase transition in ammonium hydrogenselenates, NH 4 HSeO 4 and ND 4 DSeO 4

Raman and infrared spectra of the room‐temperature ‘paraelectric’ (II) and high‐temperature (I) superionic phases of NH 4 HSeO 4 and ND 4 DSeO 4 were investigated at different temperatures in the range 300–450 K. A new intermediate phase I′ is identified on cooling phase I. A complete assignment of the bands of phase II is given and the spectroscopic manifestations of the structural disorder due to both the statistical distribution of OH … O protons and orientational disorder of ammonium ions were studied. In the superionic phase I infinite (HSeO 4 − ) n chains are broken and open dimers appear to be formed. There is increasing orientational disorder of both ammonium and HSeO 4 − ions resulting in a plastic state. Hydrogen bonding in various phases is discussed.