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

The infrared and Raman spectra of (NH4)2HPO4 and (ND4)2DPO4 were investigated between 10 and 450 K in the 4000–10 cm−1 wavenumber range. For the two compounds, four crystalline phases were evidenced and characterized spectroscopically, and structural and dynamic information was obtained. The crystalline structures and sequence phase transitions are the same for both compounds. The β phase at room temperature is strongly disordered mainly as regards the ammonium cations and a decrease in temperature leads to progressive ordering of these cations. Only the low-temperature phase δ is totally ordered. The high-temperature phase transition β α mainly involves the proton of the OH···O hydrogen bond which also becomes strongly disordered. Copyright © 2000 John Wiley & Sons, Ltd.

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