Theory of the Structural Phase Transitions in TTF–TCNQ. I

Abstract

A theory of the superlattice transitions of TTF–TCNQ is presented with strict regard for its quasi-one-dimensional characters. The model investigated is a two-dimensional array of two kinds of linear chains weakly coupled with one another. The fluctuation of phase of charge density waves on each chain is treated carefully, while the interchain couplings are approximated by mean fields. The properties of the transitions at 54 K and 49 K are well explained, and the expressions of the transition temperatures and the temperature dependence of order parameters are derived.