Theory of the Prototypic-to-Semicommensurate First-Order Transition in Ammonium Rochelle Salt on the Presumption That the Low-Temperature Phase of ARS Is Semicommensurate

It was reported that, on cooling, ARS undergoes a first-order transition from a paraelectric commensurate phase to a ferroelectric incommensurate phase. The present paper presumes the low-temperature phase to be semicommensurate, and constructs a theory of the prototypic-to-semicommensurate first-order transition. The free energy function contains term Q *5 ∂ x Q - Q 5 ∂ x Q * , which is responsible for the mentioned transition. Its effect can be greater when the first-order transition is of “type II” than when of “type I”; hence type II is assumed (what these types mean is explained). A sufficient condition for the semicommensurate phase to outrival the commensurate ferroic and onefold incommensurate phases in succeeding the prototypic phase is found out and numerically examined. The prototypic-to-semicommensurate transition is definitely possible.