The phase transition in ammonium sulphate: dynamics of deuterated NH4+ions and the heat of transition

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Abstract
The temperature dependence of the D modes of partially deuterated NH+ ions in ammonium sulphate has been investigated with a view to examining the nature of the changes in these ions. It is inferred that the changes in these ions as well as those in the H bonds at Tc are minor and gradual. In contrast, a similar study of the nu 1(SO42-) mode reveals that the SO42- ion undergoes a significant and sudden change. These observations have been used to estimate the heat of transition, giving the result 4.2+or-0.4 kJ mol-1; this agrees well with the experimentally observed values ((3.89-4.27)+or-0.02 kJ mol-1). These inferences support the assertion that a molecular-distortion-type (MD-type) microscopic mechanism is responsible for the transition.