Vibrational study of structural phase transitions in (NH4)(2)HPO4 and (ND4)(2)DPO4

The infrared and Raman spectra of (NH4)(2)HPO4 and (ND4)(2)DPO4 were invest igated between 10 and 450 K in the 4000-10 cm(-1) wavenumber range. For the two compounds, four crystalline phases were evidenced and characterized sp ectroscopically, and structural and dynamic information was obtained, The c rystalline structures and sequence phase transitions are the same for both compounds. The beta phase at room temperature is strongly disordered mainly as regards the ammonium cations and a decrease in temperature leads to pro gressive ordering of these cations, Only the low-temperature phase delta is totally ordered. The high-temperature phase transition beta --> alpha main ly involves the proton of the OH ... O hydrogen bond which also becomes str ongly disordered. Copyright (C) 2000 John Wiley & Sons, Ltd.