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

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 (F-T) in this crystal. It is reported that the transition in AS is triggered by distortion arising in the structure of the SO:-ion. This has been confirmed from a detailed analysis of the crystal structural data available at different temperatures around T,. It is concluded that the type of transition occurring in AS is different from the well known displacive/orderdisorder 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. Our model is found to be consistent with the phenomenological theory of Ikeda and co-workers which explains reasonably well the temperature variation of dielectric constant, elastic compliances, etc.

Baric Shift of Phase Transition in Ammonium Sulfate Crystals

“…It is reckoned that the PT in the AS crystal is a transition of particular type [13]. The PT in the AS is mainly defined by the SO 4 groups which change collectively their orientation, positions and the distortions at T c , thus determining a partial ordering of the ammonium groups which conserve a possibility to reorient below T c .…”

Spectroscopy of the ns1‐Centers in Ionic Crystals

“…23), resulting in a linear temperature dependence, which is indeed observed. Ammonium sulphate [(NH,),SO,] (AS hereafter) exhibits a SPT at the transition temperature 7;, = 223 K from the high-temperature paraelectric phase, which belongs to the DkE space group (isomorphous with P-K,SO,) to the low-temperature C;" ferroelectric phase [214]. Although several mechanisms were proposed, the SPT mechanism in AS still remains controversial.…”

Unmasking the Mechanism of Structural Para- to Ferroelectric Phase Transition in (NH4)2SO4

“…However, to date, the softening of the aforementioned modes has not been experimentally proven. 28,35 The most recent explanation of the AS phase transition involves the relaxation of motions of the asymmetric unit [NH 4 + (I)−SO 4 2− −NH 4 + (II)] between two possible orientations. 36−38 The phase transition was suggested to be induced by the reduction of NH 4 + (I) motions, which through the stabilization of SO 4 2− librations results in the relaxation of NH 4 + (II) hindered rotations.…”