Phenomenological Theory of the Phase Transitions in {N(CH₃)₄}₂CuBr₄

Abstract

The Landau theory is applied to investigate successive phase transitions in {N(CH₃)₄}₂CuBr₄. Transition sequence in this material from a normal phase to an incommensurate phase, followed by a commensurate ferroelectric phase and finally by the lowest temperature phase is explained. It is also shown that the phase transitions observed in {N(CH₃)₄}₂ZnBr₄ and {N(CH₃)₄}₂CoBr₄ can be discussed by the present model qualitatively.