A BEDT-TTF

Fig. 7 The novel packing motif of a" -(BEDT-TTF)4([Fe(C5H4S03)2]-6H20. Regions containing a-type circled with a dotted line, while regions with P"-packing are highlighted with an oval (top). The anionic layer is characterized by an extensive hydrogen bonding network (dashed bonds) between water molecules and the sulfonate functionality of the anion...

Several studies on systems incorporating TTF and C60 have appeared including the formation of a BEDT-TTF l3 C60 complex <00CC2357>, covalently bound TTF-C60 compounds and a C o ITF-crown ether triad . 

Tajima N, Fujisawa J, Naka N, Ishihara T, Kato R, Nishio Y, Kajita K (2005) Photo-induced insulator-metal transition in an organic conductor a-(BEDT-TTF)2l3. J Phys Soc Jpn 74 511-514... 

Katayama S, Kobayashi A, Suzumura Y (2006) Pressure-induced zero-gap semiconducting state in organic conductor a-(BEDT-TTF)2l3 salt. J Phys Soc Jpn 75 054705/1-6... 

This structure differs from those of a-(BEDT-TTF)2MHg(SCN)4 salts with respect to both the donor and acceptor sublattices [7]. The donor layers in the present salt have a P -type arrangement (Fig. 1) and are built from three different BEDO-TTF donors (A, B and C). There are three different types of intermolecular interactions with the slab of organic molecules, the relative orientation of which allows us to describe this layer as being composed of a series of parallel stacks of slipped donors along the (2a-b)-direction, as a series of step-chains along the a+2b) -direction, or as a series of parallel... 

Another demonstration of the validity of these calculations is provided by BEDT-TTF-based salts. The calculated Fermi surface of these materials exhibit closed orbits characteristic of two-dimensional electronic interactions and this has been confirmed experimentally. For example, in the case of (BEDT-TTF)2I3, the calculated surface of these orbits (Fig. 21) [61] agrees well with the one measured by magnetic experiments [161]. However, the overall good agreement between calculation and experiment must not hide the fact that some qualitative discrepancies may arise in some cases. For example, (TMTTF)2X salts exhibit a resistivity minimum at a temperature at which no structural transition has yet been observed. The resistivity minimum is not explained by the one-electron band structure, and to account for this progressive electron localization, it is necessary to include in the calculations the effect of the electronic correlations [162]. Another difficulty has been met in the case of the semiconducting materials a -(BEDT-TTF)2X, for which the calculated band structure exhibits the characteristic features of a metal [93,97,100] and it is not yet understood... 

Stereographic projections related to the plane (001) of a-(BEDT-TTF)2I3 and a, phase (Fig. 2a) and 0-(BEDT-TTF)2I3 (Fig. 2b) are shown. The indicatrix orientation of the a phase is governed by the direction L of the longitudinal axes of BEDT-TTF molecules and by the stack direction. 

Figure 2 Stereographic projection (a) related to the plane (001) of a-(BEDT-TTF)2I3 measured indicatrix orientation in the a-phase (solid line) and in the a,-phase (dashed line) principal axes X (O), Y ( + ), and Z (A) are marked (b) related to the plane (001) of 0-(BEDT-TTF)2I3 L, longitudinal T, transversal, and N, normal molecule axes I, iodine molecule axis. (From Ref. 29.)...

The structure of this salt consists of stacks of twisted dimers. It is a semiconductor with an energy gap Ec = 0.42 eV. At room temperature the electrical conductivity is crM = 0.1 S/cm and the magnetic susceptibility is X = 8 x 10-4 emu/mol. As in the preceding a -(BEDT-TTF)2X series, the magnetic susceptibility does not fit well a Bonner-Fisher law [66]. 

Fig. 3. Projections of the structuie of (top) a-(BEDT-TTF)2l3 and (bottom) P-(BEDT-TTF)2l3 along the stacking axes.

The time needed for the transformation of the crystals from a-(BEDT-TTF)2l3 to aj-(BEDT-TTF)2l3 depends on the thickness of the crystals, and strongly on the temperature, which should be between 70 C and 1(X) C. The lower the temperature, the longer the period is for the total transformation, but the quality of the crystals is much better after a transformation at lower temperatures, than after a transformation at higher temperatures. At 75 C it takes about 3 days for the transformation to be completed. In Fig. 4a an example is shown, where the transformation is not complete. The usual metal-insulator transition of the a-phase at 135 K can still be partially observed. But, at lower temperatures, the crystal shows already a... 

Since structural phase transitions during the preparation of the polycrystalline pressed samples of organic materials seem to play an important role, some additional investigations were done. Fig. 13 shows resistivity versus temperature data of a polycrystalline pressed sample of a-(BEDT-TfF)2l3, together with the resistivity characteristics of a single crystal of a-(BEDT-TTF)2l3. The polycrystalline pressed sample of a-(BEDT-TTF)2l3 still shows, in the temperature range between 300 K and 180 K, a metallic behaviour, but the sharp... 

Fig. 10. Resistivity versus temperature below 12K (normalized at 12 K) for a polycrystdline pressed sample of 0Cj-(BEDT-TTF)2l3 (prepared from a-(BEDT-TTF)2l3 and then tempered, see text) and for a crystal of at-(BEDT-TTF)2l3.

Fig. 13. Resistivity versus temperature of a poly crystalline pressed sample of a-(BEDT-TrF)2l3 and of single crystals of a-(BEDT-TTF)2l3. 

Fig. 15. Resonance Raman scattering, using different laser excitation frequencies at 1.3 K [30] of (a left) a polycrystalline pressed sample of a-(BEDT-TTF)2l3 (prepared from powdered a-crystals and then tempered), (b right) of a polycrystalline pressed sample of at-(BEDT-TTF)2l3, where the surface of the pellet was polished with a razor blade (see text).

In this paper,we report on Raman investigations of polycrystalline pressed materials of superconducting tempered a-(BEDT-TTF)2l3, called, in the following, at-(BEDT-TTF)2l3, and on untempered a-(BEDT-TTF)2l3. 

Fig. 4. Resonant Raman Scattering on the tempered polycrystalline pressed pellet of a-(BEDT-TTF)2l3 with different laser frequencies.

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