Successive Phase Transitions in Ferroelectric NaNO₂ and SC(NH₂)₂

Abstract

The successive phase transitions from a prototype to an incommensurate phase and then to a commensurate phase are studied on the basis of a model thermodynamic potential where the instability of a one-dimensional representation is assumed. The model is suitable to a unified description of the phase transitions in SC(NH 2 ) 2 and NaNO 2 which show ferroelectricity in their commensurate phases. The incommensurate states of the model and its dielectric properties are analysed by taking harmonics into account. It has turned out that the present model can satisfactorily reproduce the transitions actually observed in SC(NH 2 ) 2 and NaNO 2 .