FRONTIERS OF ORGANIC SUPERCONDUCTORS

Institute for Solid State Physics The University of Tokyo Roppongi, Minato-ku Tokyo 106, JAPAN 

There are five families of organic superconductors based on different organic moieties, namely the TMTSF (7 members), BEDT-TTF (about 13), DMET (7), MDT-TTF (1). and M(dmit)2 (4) families. The BEDT-TTF family is the biggest, and T of it is the highest. 

Some of the reasons for the occurrence of multiphase BEDT-TTF salts are the conformational freedom of the ethylene groups of the BEDT-TTF molecule, and the cooperative interactions caused by the k-k (face-to-face) overlap and sulfur-sulfur (side-by-side) atomic contact these cooperative interactions result in the 2D nature of BEDT-TTF charge-transfer (CT) complexes [18]. One of the polymorphs of P-I3 salt (a-13), has been found to change into a stable 8 K superconductor (at-13) by tempering this is considered to have a structure similar to that of high-T. P-I3 [19]. 

Since triiodide is the longest one among the symmetric linear polyhalides, we have searched for longer anions, among a variety of metal halides and metal pseudohalides, where metals are Cu, Ag, Hg, Ni, Pd, etc. and the pseudohalides are CN, OCN, SCN, SeCN, etc. In the following we describe only two BEDT-TTF complexes obtained in this process, namely superconducting Cu(NCS)2 and metallic KHg(SCN)4 salts however, neither was found to be p-phase furthermore, the anions were neither symmetric nor linear, but formed polymeric clusters. 

shiny single crystals with distorted-hexagon-shape (3 x 2 x 0.05 mm ) of k-(BEDT-TTF)2Cu(NCS)2 were prepared by the electrochemical oxidation of BEDT-TTF (prepared from CS2 by conventional methods) in 1,1,2-trichloroethane (TCE), benzonitrile or THF in the presence of (1) CuSCN, KSCN and 18-crown-6 ether, (2) K(18-crown-6 ether) Cu(NCS)2 or (3) CuSCN and TBA-SCN. For electrolytes (1) or (3), undissolved materials remained on the bottom of the cell during the course of electrocrystallization, but the precipitation did not affect the crystal growth. Crystals were grown in H cells (total volume ca. 20 mL) or modified H cells, where one cell compartment is an Erlenmeyer flask (total volume ca. 100 mL). The anode and cathode were separated by a medium-porosity frit. 

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