Phase transition and temperature dependence of the molecular distortion of ions in ammonium sulphate

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Abstract
The temperature dependence of the molecular distortion of ions in ammonium sulphate (AS) is discussed in order to ascertain the correct microscopic mechanism of phase transition (PT) in this crystal. It is reported that the transition in AS is triggered by distortion arising in the structure of the SO42- ion. This has been confirmed from a detailed analysis of the crystal structural data available at different temperatures around Tc. It is concluded that the type of transition occurring in AS is different from the well known displacive/order-disorder type and it could be given a name such as molecular distortion type. Such a transition can only take place in crystals having one or more than one kind of molecular unit. It leads to a change mainly in the structure and symmetry of such units, rather than in their location and orientation. This model is found to be consistent with the phenomenological theory of Ikeda and co-workers (1973) which explains reasonably well the temperature variation of dielectric constant, elastic compliances, etc.