TMTTF

Saito et at.130 studied the salts of TMTSF and the sulfur analogue tetra-methyltetrathiafulvalene, TMTTF, with a polycyano dianion. Although the conductivity of both compounds was low (crrt = 10-5 Scm-1 for TMTSF vs. 10-7 Scm-1 for TMTTF) the conductivity of the Se-donor salt was improved by two orders of magnitude. Optical absorption spectroscopy was also used to assess the materials. The electronic transition between radical cations within the segregated donor columns occurred at considerably lower energy (8800 cm-1) in the TMTSF salt than in the TMTTF (11500 cm-1). A concurrent improvement... 

Dlh TTF, TCNQ, TMTTF, TMTSF, BEDT-TTF, PTCDA, Ni(dmit)2, BEDT-TSF, BDA-TTP, pentacene... 

CIO4 ion being smaller than the octahedral PFg ion, the TMTSF sheets become closer to each other, chemically inducing the same effect as the physical hydrostatic pressure. When replacing TMTSF by TMTTF one obtains the isostructural Fabre salts. 

We saw in Section 1.1 for N2 that electron localization implies insulating ground states and that localization can be reduced by applying external pressure. When reducing temperature from RT down to about 20 K, (TMTTF)2X and (TMTSF)2X... 

Figure 1.19. Generic T-P phase diagram for BFS. The origin on the pressure axis is arbitrarily set for (TMTTF)2PF6. MH, Mott-Hubbard M, Metal SP, Spin-Peierls AF, Antiferromagnetic SDW, Spin-Density-Wave SC, Superconductor. Adapted from Auban-Senzier J6rome, 2003.

Figure 1.30. Theoretical band structure of (TMTTF)2Re04 along the F-X and F-Y directions. Courtesy of Dr E. Canadell.

One way of experimentally exploring the electronic structure of solids is by means of photoemission spectroscopies such as UPS and X-ray photoelectron spectroscopy (XPS), where photoexcited electrons are analyzed dispersively as a function of their kinetic energy. The electronic structure of the reference material TTF-TCNQ will be extensively discussed in Section 6.1. Figure 1.31 shows the XPS spectra of the S2p core line for (TMTTF)2PF6 (black dots) and BEDT-TTF (grey dots). 

The (TMTTF)2PF6 spectra can be satisfactorily decomposed into two main lines, each with their corresponding spin-orbit doublet, the lower energy line coinciding... 

Figure 5.15. XRD pattern of a 1 p.m thick TMTTF/NaCl(100) film grown by PVD. The (200) reflection from the substrate is intentionally only partially shown.

XRD patterns of TTF-TCNQ hlms grown by CVD on Si(lOO) substrates also show this kind of extra reflections (de Caro et al, 2000a). This is perhaps the first evidence, albeit incomplete and thus questionable, of a new crystallographic phase of TTF-TCNQ. The conclusive observation of a new phase of TTF-TCNQ, e.g., with mixed-stacked structure as for the red phase of TMTSF-TCNQ, would be extremely interesting. The known and newly observed structures for both TMTTF and TTF-TCNQ might be tentatively ascribed to the thermodynamical and kinematical phases, respectively. 

Figure 5.16. XRD patterns of um thin TMTTF films grown by PVD on (a) OTS, (b) BaAA and (c) Si02 substrates.

In agreement with this analysis, single-crystal conductivity measurements using the four-probe technique reveals semiconducting behaviour for /r -(TMTTF)2Re04, as shown in Fig. 6.30. In this case aRj — 0.011 cm and Ea — 0.17 eV. 

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