Model for the low-temperature phases and the two phase transitions of tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ)

The evidence concerning the 38- and 53-K phase transitions in tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ) and the phases below each is critically reviewed. It is concluded that the 53-K transition is on the TCNQ stacks, that the TTF stacks show negligible long-range order. A Ginzburg-Landau model is constructed that assumes that isolated TCNQ stacks have a much stronger tendency to a Peierls transition and much larger amplitude charge-density waves than do the TTF stacks. Coulomb interactions between charge-density waves are assumed to be the dominant interstack interaction. The model shows how the TTF and TCNQ stacks are largely decoupled from one another and how the development of TTF long-range order can change the superlattice cell parameter along $\overrightarrow{\mathrm{a}}$ from $2a$ at 53 K to larger values at lower temperatures. The model also has implications for the electric and magnetic behavior.