TTF salts

Scheme 5 Electronic structures of TTF salts, showing charge localization into uniform chains or dicationic dimers in the TTF+ X salts (left) or the partially filled conduction bands in mixed-valence (TTF X salts (right)...

TTF salts containing similar ferrocenium anions have been prepared through metathesis reactions between (TTF)3(BF4)2 and the tetraphenylphosphonium salts of the desired anions [71]. The crystal structure of (TTF)(CpFeCp-CONHCH2 S03)-1/3H20 contains isolated TTF dimers. Hydrogen bonds of the —NH—OS- and NH—0=C- type are present between anions. Two phases of the TTF salts with the... 

S 3p orbital to the HOMO of DMET or EDTDM molecule. Therefore It J and also I JndI of the EDS-TTF salt becomes negligibly small compared to the DMET or EDTDM salt. It should also be noted that the intermolecular Br—Br distance between FeBr4 anions is almost the same among the EDS-TTF, DMET and EDTDM salts. Therefore this also proves the importance of the n-d interaction in the long-range magnetic ordering of the -electron spins. 

From work on crystalline TTF salts in the solid state 15,16 these two new absorptions can be assigned as the dication dimer, D22+, absorbing at O.8/1,... 

BTDM-TTF is one of the most powerfnl donors of the TTF class. Alkylthio-substitution, such as in BEDT-TTF (ET), leads to a slight redaction in its electron-donor ability. Nevertheless, salts of this cation-radical with tetracyanoqninodimethide (TCNQ) and its derivatives show significantly higher conductivity (by ca. 10 -10 ) than the analogons BTDM-TTF salts (Grossel and Weston 1994). This reflects some enhancement of intra and interstack interactions produced by the sulfur atoms at the edges of the donor skeleton. 

Materials with the general formula (TMTCF)2X, where C stands for chalcogens sulfur and selenium and X for monovalent anions, are known as Bechgaard-Fabre salts (BFS). Because of the extremely high quality of the BFS that can be achieved, these are, together with TTF-TCNQ and BEDT-TTF salts, the most extensively studied crystalline MOMs and a matter of intensive research. 

This work shows the exceptional physics that can be done with a STM operated at cryogenic temperatures and the availability of STMs working down to liquid helium temperature opens broad avenues of research in the coming years. No doubt that among the many future scientific experiments accessible with low temperature STMs, the real-space electronic characterization of the metal-superconductor transition in /c-phases of BEDT-TTF salts, because Tc > 4 K, as well as the study of magnetic ordering in MOMs, will certainly occupy a relevant position. 

So far all TMTSF and almost all ET-type superconductors have mixed valence and p=. The TMTSF salts and some ET salts are quasi-ID, but with enough warping of the Fermi surface to make them pseudo-2D and to defeat the Peierls transition. The k-phase (BEDT-TTF) salts are really 2-D systems this phase, shown in Fig. 12.7, has isolated dimers connected by dispersion interactions to form, roughly, two-dimensional sheets. Figure 12.8 shows the superconductivity of the salt k-(BEDT-TTF)2Cu(NCS)2 at 10.4 K [34]. 

The second family of superconducting materials is based on cation-radical salts of another donor, bis(ethylenedithio)-TTF (abbreviated BEDT-TTF or ET), which exhibit two-dimensional network mostly due to inter-stack S S interactions [100]. Another important feature of BEDT-TTF salts is their tendency to give polymorphs. For instance, (BEDT-TTF)2I3 salt affords four polymorphs (a, p, 0, and k phases), of which only the first undergoes metal-insulator transition, while the others are superconductors at ambient pressure (see Chapter 10 of this book). It is quite surprising that of all numerous BEDT-TTF structural analogs synthesized to date, only two salts of unsymmetrical derivatives, DMET [101] and MDT-TTF [102], led to superconductors. 

Electrocrystallization can be conducted under conditions of constant current or constant voltage. The former is more common. In a constant-current experiment, the initial current density should be low and increased as required. For BEDT-TTF salts, a starting current density of about 0.1 pA/cm2 is reasonable. Optimum current densities are usually in the range... 

Similar examinations of the CT spectra for bis(propylenedithio)-tetrathiafulvalene (BPDT-TTF) salts [36], BEDT-TTF salts [37], and bis-tetramethylenetetraselenafulvalene-(4,5-dimercapto-l,3-dithiole-2-thione)nickel [OMTSF-Ni(DMIT)2] salt [38] have been performed. The latter salts are examples of organic conductors that are almost isotropic in two dimensions. Thus only weak polarization dependence is found in the entire frequency range. The analysis of the spectra within a simple DA-charge oscillator model, which takes into account the coupling to intramolecular vibrational modes, demonstrates how IR and optical measurements can provide estimates for a number of physical parameters for lowdimensional organic conductors. 

The BEDT-TTF trihalides and the related salts attract much attention because of a relatively high superconducting transition temperature. Figure 8 shows the polarized reflectance of a- and (3-(BEDT-TTF)2I3 crystals for two light polarizations. For both phases the electronic reflection bands with a Drude-like edge are observed in two perpendicular polarizations [47]. Drude parameters and transfer integrals of typical (BEDT-TTF)2X salts are 5000 cm-1 < top < 9600 cm-1, 500 cm-1 < y < 2000 cm-1, and 0.08 eV < t < 0.20 eV. Near isotropy of the optical properties of typical BEDT-TTF salts is confirmed by electrical transport studies. Rather small values of t are consistent with relatively low room-temperature conductivity. 

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