Phenomenological theory of the phase transition in improper ferroelectric ammonium sulphate

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Abstract
A phenomenological theory accounting for the phase transition in improper ferroelectric ammonium sulphate has been discussed that is based on a molecular distortion-type microscopic mechanism (reported earlier). The theory uses the SO42--ion distortion as an order parameter and considers its linear coupling with the spontaneous polarisation to account for the ferroelectric behaviour. It has been used to account for the temperature dependence of the order parameter and dielectric constants. The dielectric susceptibility and the coefficient of linear coupling between the order parameter and the spontaneous polarisation in the ferroelectric phase are found to be different from those in the para-electric phase. The temperature dependence of the spontaneous polarisation and its possible origin have also been discussed.