A Model of Successive Transitions in {N(CH3)4}2ZnCl4

On the basis of a model free energy, mechanisms of the successive transitions from a normal phase to an incommensurate one, a commensurate one and finally to some low temperature phases are investigated. The free energy consists of terms of six physical quantities, and is invariant in the normal phase Pmcn . It is shown that the incommensurate phase transforms to the commensurate ferroelectric phase with fivefold cell-length. Further the temperature is decreased, then a triple cell-length phase is realized. The present model is appropriate for {N(CH 3 ) 4 } 2 ZnCl 4 and {N(CH 3 ) 4 } 2 CoCl 4 , for example, and differs from those proposed for K 2 SeO 4 and the isomorphous materials.