A landau theory for the displacive phase transitions in TTF-TCNQ

Abstract

Translational and librational modes of TTF and TCNQ molecules are treated within the framework of classical Landau theory. The various parameters are estimated from the crystalline structure. The Coulomb forces between TTF sulphurs and TCNQ nitrogens are assumed to be dominant, and this approximation establishes a hierarchy of interactions, which in turn gives rise to a cascade of displacive transitions, in excellent agreement with experiment. The change in transverse period of the periodic lattice distortion from 2 α to 4 a between 48 K and 38 K is accounted for, and found to be due to the staggered placement of the nitrogens with respect to the sulphurs. The X-ray reflections with wavevector 4 kF are attributed to librations possessing a glide-plane symmetry element, which suppresses the 2 k F reflections. Symmetry prohibits these reflections from being coupled (in first order) to the Peierls transition on the same chain, however they couple strongly to the Peierls distortion on the neighbouring chains.