**2. Preparation of organic superconductors**

106 Superconductors – Materials, Properties and Applications

CT solids from the 1960s [7]. The first metallic CT solid TTF•TCNQ appeared in 1973 [8] based on the two main requirements for the conductivity, namely, (1) a uniform segregated stacking of the same kind of component molecules, and (2) the fractional CT state (uniform partial CT) of the molecules. Since TTF•TCNQ has a low-dimensional segregated stacking, it showed a metal-insulator (MI) transition (Peierls transition) below about 60 K. For TTF•TCNQ, the Peierls transition occurs by the nesting of the one-dimensional Fermi surface causing lattice distortion associated with the strong electron-phonon interaction and forms charge density wave (CDW). There are also several one-dimensional organic metals which show MI transitions by the formation of spin density wave (SDW) when the periodicity of the SDW coincides with the nesting vector of Fermi surface and no lattice distortion occurs in this case. An increase in the electronic dimensionality is inevitable to prevent the nesting of Fermi surfaces and develop superconductors. Several attempts have been made through "pressure", "heavy atom substitution", or "peripheral addition of alkylchalcogen groups" (Fig. 1). The latter

two correspond to the enhancement of the self-assembling ability of the molecules.

Appropriate examples taking TTF derivatives are shown in Fig. 1. Based on TMTSF molecules several superconductors under pressure have been prepared with warped one-dimensional Fermi surface since 1980 (a in Fig. 1) [9–14]. In general, the ratio of transfer energies (*t*// / *t*⊥) is larger than 3 for one-dimensional Fermi surface and a closed two-dimensional Fermi surface is formed when *t*// ≤ 3*t*⊥, where *t*// and *t*<sup>⊥</sup> are the transfer energies along the directions of the largest and second largest intermolecular interactions. The BO (BEDO-TTF) molecules afforded stable two-dimensional metals having two-dimensional Fermi surface (b in Fig. 1) owing to the strong self-assembling ability by intermolecular S∙∙∙S and hydrogen-bonds [15], and only two superconductors are known since 1990 (*T*<sup>c</sup> ≤ 1.5 K). The substitution of an ethylenedioxy group with an ethylenedithio group (BO → ET (BEDT-TTF)) destabilized the metallic state of BO compounds and provided unstable two-dimensional conductors (c in Fig. 1). Consequently, variety of superconductors and other functional solids have been developed based on twodimensional metals of ET since 1982 (*T*<sup>c</sup> ≤ 13.4 K) [16–20] and its analogues (*T*<sup>c</sup> ≤ 10 K) [21].

**Figure 1.** Strategy for chemical modification of the TTF molecule to increase (arrows) or decrease (dotted arrow) the electronic dimensionality by the aid of enhancement or suppression of the selfassembling ability of the molecules, respectively [16]. Typical Fermi surfaces of TMTSF (a:


β

(TMTSF)2NbF6), BO (b: (BO)2.4I3), and ET (c:

CT solids are prepared mainly by the following three redox reactions: (1) electrocrystallization (galvanostatic and potentiostatic), (2) direct reaction of donors (D) and acceptors (A) in the gaseous, liquid, or solid phase, and (3) metathesis usually in solution (D•X + M•A → D•A + MX, M: cation, X: anion). In the latter two cases, single crystals are produced by the diffusion, concentration, slow cooling, or slow cosublimation methods.

Electrocrystallization (main procedures in detail and corresponding references are described in Section 11 of Ref. 17) is performed with a variety of glass cells, as shown in Fig. 2. Strictly speaking, the potentiostatic method is the proper way, in which a three-compartments cell is employed and one of the compartments contains the reference electrode, such as saturated calomel or Ag/AgCl electrode. However, this method is troublesome when a large number of crystal-growth runs are performed for a long period of time due to the following: 1) the contamination through the use of a reference electrode cell, and 2) the limited space for the experiment. The galvanostatic method is much more convenient than the potentiostatic one from these points of view. An H-cell (20 ml or 50 ml capacity) and an Erlenmeyer-type cell (100 ml) with a fine- porosity glass-frit equipped with two platinum wire electrodes (1–5 mm in diameter) have been used (Fig. 2).

There are many factors and tricks to grow single crystals of good quality. The important factors besides both the purity and the concentration of the component materials are the kinds of solvent and electrolyte, the surface of the electrode, the current (0.5–5 μA), and temperature. THF (tetrahydrofuran), CH2Cl2, TCE (1,1,2-trichloroethane), chlorobenzene, CH3CN, and benzonitrile are commonly utilized solvents. The addition of 1–10 v/v% ethanol occasionally accelerates the crystal growth. As for the electrolyte, solubility in organic solvent is an important factor and usual electrolytes are tetrabutylammonium (TBA) or tetraphenylphosphonium salt of anion X. Sometimes, the electrolyte is a combination of

soluble and insoluble materials. For example, single crystals of κ-(ET)2Cu(NCS)2 were prepared using 1) CuSCN + KSCN + 18-crown-6-ether, 2) TBA•SCN + CuSCN, or 3) Cu(NCS)2•K(18-crown-6-ether). Low solubility of the components of the electrolyte in the specific solvent usually retarded single crystal growth. Ionic liquids such as 1-ethyl-3 methylimidazolium (EMI, Scheme 2) salts of X were found to afford single crystals of high quality, recently. Regarding the surfaces of electrodes each research group has special treatments such as burning (but not melting) or polishing with very fine powder. The electrode surface can be treated by applying a current to switch the polarity in a 1 M H2SO4 solution. When the radical species are unstable in solution, CT solids can be grown by applying a high current at very low temperatures; *e.g.*, salts of fluoranthene (–30 °C, 2 mA, Ni electrode), naphthalene, and azulene.

Development and Present Status of Organic Superconductors 109

The earliest route for reductive intercalation of C60 solids by alkali or alkaline-earth metals is the vapor-solid reaction by vacuum annealing. Almost all superconductors based on graphite and polyaromatic hydrocarbons have been obtained in accordance with this synthetic route. Besides pristine alkali metals, sodium mercury amalgams, sodium borohydride, alkali azides, and alkali decamethylmanganocene have been utilized for source of alkali metal vapors to reduce C60 solids. Disproportionation reaction between C60 and M6C60 (M: alkali metal) has been sometimes utilized to obtain superconducting M3C60 or non-superconducting M4C60. For a low-temperature solution route, liquid ammonia and methylamine have been sometimes utilized for the reaction media. Especially, Cs3C60 with the highest *T*c among the C60 superconductors can be obtained only when the stoichiometric amounts of cesium metal and C60 were reacted in the dissolved methylamine media (see Section 3-2), while the conventional vapor-solid reaction gives energetically stable Cs1C60

and Cs4C60 instead of Cs3C60 with nominal composition.

**3.1. Superconductors based on electron donors** 

*3.1.1. One-dimensional superconductors (TMTSF and TMTTF families)* 

**(a) (b) (c) (d)**

TMTSF [9–14] has provided eight quasi-one-dimensional superconductors; (TMTSF)2X with highest *Tc* ~ 3 K [11] (Table 1). Most of them were prepared by electrocrystallization using TBA•X as electrolyte except the NbF6 salt which can be only prepared by using ionic liquid EMI∙NbF6 [12]. They are isostructural to each other and the crystal structure of (TMTSF)2NbF6 is depicted in Fig. 3, where TMTSF molecules form a zigzag dimer that forms a segregated column along the face-to-face direction (*a*-axis) with no short Se∙∙∙Se atomic contacts (Fig. 3a,3b). Along the side-by-side direction (*b*-axis), rather short Se∙∙∙Se atomic contacts were seen (Fig. 3c), however, those less than the sum of the van der Waals radii (3.80 Å) are present only for X = ClO4 and FSO3. For the PF6 salt, *t*a and *t*b were estimated to be 0.25–0.30 eV and 0.031 eV, respectively. Consequently, the Fermi surface of (TMTSF)2X is not closed, but open with fair warping due to the lack of adequate side-by-side transfer

**Figure 3.** Crystal structure of (TMTSF)2NbF6 [12]. (a) Segregated column of TMTSF molecules. The numbers indicate the overlap integrals in 10–3 units. (b) Zigzag stacking of the TMTSF column. (c) Se∙∙∙Se atomic contacts (*d*7, *d*9) along the side-by-side direction. (d) Temperature dependence of resistivity

**3. Structures and properties** 

interactions (Fig. 1a).

under pressure.

**Figure 2.** (a) Galvanostatic electrocrystallization using 20 ml cells on the desk. Under the desk the diffusion method is seen. (b) Single crystals of κ-(ET)2Cu(NCS)2 on the electrode in 100 ml cell and (c) showing two-dimensional conducting plane (*bc*). (d) Single crystal of (TMTSF)2ClO4 with four gold wires connected by gold paste. (e) Typical glass cells for electrocrystallization.

### **Scheme 2.**

Besides the electrocrystallization, superconducting single crystals of good quality were prepared by direct chemical oxidation of ET with iodine in gas or with TBA•I3 or TBA•IBr<sup>2</sup> in solution. Better-quality single crystals of (TTF)[Ni(dmit)2]2 were obtained by the diffusion method of the metathesis reaction rather than electrocrystallization. No single crystals of the electron acceptor based superconductors were obtained except for the M(dmit)2 system.

The earliest route for reductive intercalation of C60 solids by alkali or alkaline-earth metals is the vapor-solid reaction by vacuum annealing. Almost all superconductors based on graphite and polyaromatic hydrocarbons have been obtained in accordance with this synthetic route. Besides pristine alkali metals, sodium mercury amalgams, sodium borohydride, alkali azides, and alkali decamethylmanganocene have been utilized for source of alkali metal vapors to reduce C60 solids. Disproportionation reaction between C60 and M6C60 (M: alkali metal) has been sometimes utilized to obtain superconducting M3C60 or non-superconducting M4C60. For a low-temperature solution route, liquid ammonia and methylamine have been sometimes utilized for the reaction media. Especially, Cs3C60 with the highest *T*c among the C60 superconductors can be obtained only when the stoichiometric amounts of cesium metal and C60 were reacted in the dissolved methylamine media (see Section 3-2), while the conventional vapor-solid reaction gives energetically stable Cs1C60 and Cs4C60 instead of Cs3C60 with nominal composition.
