Theory of the Transition from a Prototypic Phase to a Simultaneously Incommensurate and Ferroelectric Phase

Abstract

Although the transition sequence [prototypic phase→incommensurate nonferroelectric phase→commensurate ferroelectric phase] is more common, the transition [prototypic phase→incommensurate ferroelectric phase] is also possible. A thermodynamical theory of this transition is presented. Ammonium Rochelle salt is dealt with as an example. Not only the soft order parameters having wavevector (1/2, 0, 0) for translational transformations, but also a hard order parameter transformed like the (y, z) component of strain is taken into account. Their cooperation is important. It is found that, if the coefficients in the free energy function satisfy certain inequalities, the prototypic phase is changed to an incommensurate ferroelectric phase earlier than to any other ferroic phase.