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Far infrared reflectivity and transmission measurements of para-and ferroelectric ammonium sulfate (NH4)2SO4 in the spectral region 30-400 cm-1 and the temperature region 100-300°K were performed in the polarized light. The reflectivity spectra were subject to the Garners-Gonig analysis and the results were discussed with the help of the group analysis and structural considerations. The dynamic nature of the phase transition is discussed in terms of a coupled oscillator-relaxator model and the order-disorder mechanism is confirmed. The concept of a pseudo-proper ferroelectric is discussed and it is shown that the ammonium sulfate is a typical representant of this class of ferroelectrics.
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Petzelt et al. (1973) studied this question.
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