TMTSF crystal structure

Crystal structure of (TMTSF)2CI04 [28], projected along [010], The TMTSF stacking is vertical the stacks are separated by perchlorate anions at the corners of the unit cell. 

Figure 1 Prototype molecules TTF (tetrathiafulvalene and TMTSF (tetramethyl-tetraselenafulvalene) which are used for the elaboration of organic conductors TTF-TCNQ or superconductors (TMTSF)2X. View of the crystal structure of some molecular superconductors (a) (TMTSF)2PF6 (b) (alkali)3 C (c) (BEDT-TTF)2Cu(SCN)2 side view (d) view along the axis perpendicular to the conducting plane.

The crystal structure of (TMTSF)2Br04 is shown in Fig. 5 as representative of the series (32). The basic architectural feature of the isostructural (TMTSF)2X salts is the zig-zag columnar stacking of nearly planar TMTSF molecules parallel to the high-conductivity a axis (10, 11, 32-40). The structures of the salts are different from that of... 

Fig. 10. The H-bonding environment (stereoview) about the AsF6" anion, derived from the low-temperature (125 K) X-ray crystal structure of (TMTSF)2AsF6, is far more symmetrical than that of the C104 anion (39). For clarity the AsF6 anion has been drawn in an ordered configuration with the fluorine atoms occupying six positions. A disordered model with 12 partially occupied fluorine positions gives similar results.

Fig. 11. A stereoview of the ordered CIO - anion environment in the low-temperature (125 K) X-ray-determined crystal structure of (TMTSF)2C104. Short H2C-H - 0-C103 hydrogen bonding interactions (drawn as faint lines for 0 - - H < 3.0 A) exist for 0(1) and 0(2), which limit the thermal motion of these atoms and may be responsible for pinning the C104 anion in the lattice (53).

Turning now to a discussion of the crystal structures of (ET)2X conductors, we start by noting that these do not always crystallize with one single type of structure. Therefore, at this time, it is not feasible to carry the analysis of their structure-property relationships to nearly the same degree of detail as was done for the (TMTSF)2X series. 

In conclusion, XDS studies of numerous TMTSF systems have provided a great deal of information on the nature of the anion-ordering transitions that occur at various temperatures. However, detailed single-crystal structural analyses are still required in order to determine the precise structural changes associated with these transitions. 

X represents a monovalent anion. Unlike (TMTSF)2X, the crystal structure of the ET salts can vary a lot. A few examples will be presented in the next section. 

By using the donor TMTSF (tetramethyltetraselenafulvalene, 11b) and the newly developed technique of electrocrystallization, the groups of Bechgaard and Jerome found in 1990 the first organic superconductor, (TMTSF)2PF5, with Tj = 0.9 K at an applied pressure of 10 kbar [29]. Here the lateral two-dimensional Se-Se interactions and the applied pressure together defeat the Peierls transition. The first ambient-pressure superconductor, (TMTSF)2C104, followed quickly Fig. 4 shows its crystal structure. 

The crystal structures of the two systems are also very alike. Both are triclinic with comparable unit-cell parameters and with the "herring-bone" structure known from TTF-TCNQ /8/. The most pronounced crystallographic difference between the two systems is a rather elongated /6.1 %/ b-axis in TMTSF-DMTCNQ. This is to be expected because the b-direc-tion is across the width of the molecules where the additional methyl-groups tend to push the chains apart thereb. y probably reducing the interchain coupling. 

Fig. 10.2 The crystal structure of the Bechgaard salt (TMTSF)2PF6, the first organic superconductor. The H atoms are - as usual -left off for clarity. The PFg anions are arranged between the stacks of the organic molecules. The perpendicular spacing within the stacks...

Since the first discovery of superconductivity in an organic material [1], a number of organic superconductors have been found in the TMTSF and BEDT-TTF families. However, there are many differences in the crystal structures and physical properties between the two families. Recently we have discovered seven organic superconductors in the family of the unsymmetrical donor DMET [2-6]. These new superconductors are the first ones based on an unsymmetrical molecule. They are also useful for understanding the organic superconductors systematicSly, because the DMET salts seem to link the TMTSF and BEDT-TTF families DMET has halves of the structures of TMTSF and BEDT-TTF. 

TMTSF salts, another is similar to the BEDT-TTF salts [7,8]. In this report we summarize the crystal structures and the physical properties of DMET superconductors. 

The crystal structure of (DMET)2l3 is very similar to that of (DMET)2Au(CI 2 [15,16]. However, these salts are different from each other, because (DMET)2Au(CN)2 is similar to (TMTSF)2X, while (DMET)2l3 is similar to p-(BEDT-TTF)2X. The difference in properties... 

Figure 10.3 Crystal structures of (TMTSF>2X (a) X= CIO4 at RT (CIO4 is disordered) (b) X = Re04 below 180 K (Re04 is ordered) (c) X = PFg at RT (the dashed lines indicate intercolumn SeLSe atomic contacts, Se atoms are shown by closed circles) (d) Fermi surface of (TMTSF)2X. Arrows indicate the nesting vector.

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