TTF stacks

Figure 2.. Polarized reflectivity spectra of p-BEDO-TTF)5[CsHg(SCN)4]2 for E 1 L and E L at 300, 200, 100 and 10 K. (L is BEDO-TTF stack direction). The fit with Drude-Lorenz model for T=10 K is shown by thin solid line.

At high temperature, TTF TCNQ is metallic, with a(T) oc T-2 3 since TTF TCNQ has a fairly high coefficient of thermal expansion, a more meaningful quantity to consider is the conductivity at constant volume phonon scattering processes are dominant. A CDW starts at about 160K on the TCNQ stacks at 54 K, CDW s on different TCNQ chains couple at 49 K a CDW starts on the TTF stacks, and by 38 K a full Peierls transition is seen. At TP the TTF molecules slip by only about 0.034 A along their long molecular axis. 

An important distinction must be made between the two-stack systems (like TTF TCNQ, where electrons travel on the TCNQ stacks while holes live on the TTF stacks) and the one-stack systems, or ion-radical salts (or radical ion salts), such as the (TMTSF)2X salts, the alkali TCNQ salts M (TCNQ), and the (ET)2X salts The holes are localized on the TMTSF or ET sublattice, whereas the electrons are on the TCNQ sublattice. 

Although isostructural to the Au complex, the Ni and Pt salts do not form spin-ladder systems. This may be due to slight differences in the transfer integrals within the DT-TTF stacks and also to the paramagnetism of [M(mnt)2] (M = Ni, Pt) that may interact with the DT-TTF system. Related Co and Fe compounds have also been reported (311), but they are just simple ionic salts as shown by their 1 1 stoichiometry (instead of the 2 1 stoichiometry for the Au, Ni and Pt compounds). Very recently, Ribera et al. (307) published an excellent and detailed report on this family of compounds. 

Figure 4 Room-temperature crystal packing of TTF-TCNQ. (a) View normal to the (ac) plane. (From Ref. 35.) (b) Schematic representation of the structure (shaded rectangles TCNQ white rectangles TTF). The molecular planes are tilted about the a axis there are two identical molecules with opposite tilts for one repeat unit along c, and TCNQ and TTF stacks alternate in the a direction.

The 4kF instability gives rise to the only detectable superstructure at 300 K. The IR properties of the 4kF CDW consist of a maximum in cr(co), corresponding to a pseudogap, plus a number of sharper features near the ag vibrational modes. It was shown [75] that the T dependence of the vibrational modes of TCNQ follows that of the 2kF scattering. Thus the 4instability must take place on the TTF stacks. This conclusion agrees with other studies of TTF-TCNQ salt. 

Two chain compounds, such as TTF-TCNQ and its derivatives, where both donor (TTF) stacks and acceptor (TCNQ) stacks give comparable contributions to the electrical conductivity. Although these compounds... 

The Valence and Conduction Bands of TCNQ and TTF Stacks and of the Four Homopolynucleotides (in eV-s)... 

The method has been applied to polyene, polyacetylene and polydiacetylene chains, to formamide chains (both hydrogen-bonded and stacked). Applications have been done also to TCNQ and TTF stacks, to (SN) and to periodic DNA models (the four homopolynucleotides), to the sugar phosphate chain of DNA and to different periodic protein models (homopolypeptides). All these systems have relatively broad valence and conduction bands (bandwidths around or larger than 0.5eV) according to our results. 

In the diffuse X-ray measurements of TTF-TCNQ the superlattice reflection was found with the wave number 4kp = 0.59 b [56]. It is observed even at room temperature and suggests the absence of the interchain correlation above 49 K. A set of superlattice reflection was found below 49 K suggesting the formation of an ordered structure of three-dimension. This superstructure is ascribed to the molecular displacement caused by the Wigner crystal of electrons through the electron-lattice interaction [67]. The 4 p structure is considered to be formed predominantly on the TTF stacks. The 2kp superstructure is rather ascribed to TCNQ stacks. This is suggested [68] by detailed analyses of the results of X-ray, neutron, EPR and NMR measurements. 

Figure 2 Room-temperature crystal structure of TTF TCNQ [188]. The upper diagram is projection along [ 100], with the unit cell axes a vertical and c horizontal, which shows the stacks of TTF (open circles for the atom positions) and TCNQ (filled circles for atom positions). The lower diagram shows the intermolecular overlap, projected normal to the least-squares molecular planes, along the TTF stack (left), and the TCNQ stack...

Among the [M(dmit)2]-based superconductors, a-[EDT-TTF][Ni(dmit)2] is also of DA type and has two outstanding features it is the oiJy one to contain a 1 1 molar ratio of donor and acceptor units and to exhibit superconductivity at ambient pressure. The phase is superconductive below 1.3 K under ambient pressure (Figure 4.24). a-[EDT-TTF] [Ni(dmit)2] exhibits a unique metallic behaviour with a characteristic resistivity peak at around 14 K. Magnetoresistance studies shown that the conduction mainly takes place along the Ni(dmit)2 stacks below 10 K, while both [Ni(dmit)2] and [EDT-TTF] stacks are involved above... 

We propose in this paper that the observed Peierls instability in TTF-TCNQ and the richness of structural transitions arises from the interaction of the tt electron and hole system on separate TCNQ and TTF chains, respectively, with orientational modes (librons) of the solid. We suggest, that the chiral charge density waves (CCDW), which result from the orientational distortion of the TCNQ and TTF stacks account for the observed continuous increase in the unit cell dimension from a = 2a at 54° to a = 4a at 38°K. 

What may seem surprising is that a TSeF impurity, while presumably inducing only a short-range charge density wave on its TTF stack is able to have as large an effect as it does on the TCNQ stacks. That such an effect should occur is consistent with the notion, which will be pursued... 

Perturbations in the TCNQ stacks have big effects on the TTF stacks, comparable with the effects of perturbations directly in the TTF stacks. 

This is in marked contrast to the effect of TTF stack perturbations on the TCNQ stacks. This finding can be explainable in terms of the following model ... 

Fig. 9 Top chemical structure of the TTF derivative. Middle cartoon of the TTF stacking. Bottom STM image of the TTF derivative at the HOPG-octanoic acid interface. (Reproduced with permission from [25])...

Fig. 2.17 Schematic of the TTF-TCNQ structure (see also Fig. 2.8). The molecular planes are not perpendicular to the stacking axis b here, but rather are slightly rotated towards the a axis. Along the a direction, the TCNQ and the TTF stacks alternate.

The second chapter examines applications of crystal-orbital theory presented earlier. These applications include polymers widely used in the production of plastics, primarily polyethylene and its fluorine derivatives. Other examples are from the field of highly conducting polymers, such as the different polyacetylenes, (SN) , and TCNQ and TTF stacks. Applications to nucleotide base stacks and to periodic polynucleotides and periodic polypeptides conclude this part of the book. 

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