THEORY OF THE PROTOTYPIC-TO-SEMICOMMENSURATE 1ST-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 ∂xQ-Q5 ∂xQ*; 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. © 1990, THE PHYSICAL SOCIETY OF JAPAN. All rights reserved.