Low-Temperature Studies. IV. The Phase Transitions of Ammonium Sulfate and Ammonium-d4 Sulfate; the Nature of Hydrogen Bonding and the Reorientation of the NX4+ Ions

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Abstract
The temperature dependence of the infrared spectra of (NH4)2SO4 and (ND4)2SO4 between 17 and 298°K has been accurately determined. An analysis of the spectra of the high‐temperature and ferroelectric phases of (NH4)2SO4 supports the evidence of the neutron‐diffraction data of Schlemper and Hamilton that there is an increase in hydrogen bonding below 223°K. From the temperature variation of the peak heights of certain bands it is shown that graphs of this type can be used to determine the presence of any phase transitions accurately. Inflection points of these curves of (NH4)2SO4 at ∼ 160°K are correlated with the NMR linewidth transition at 163°K; it is the first time that such a correlation has been established. The temperature dependence of ND4+ stretching bands indicates the possible existence of three phases in (ND4)2SO4 (transition temperatures 225°K and ∼ 223°K). The torsional mode of the NH4+ ions has been identified; there is no evidence that the ion rotates “freely” at any temperature.