NEW ORGANIC SUPERCONDUCTOR, LAMBDA-(BETS)(2)GACL4 AND METAL-INSULATOR-TRANSITION OF BETS CONDUCTOR WITH MAGNETIC ANIONS (BETS=BIS(ETHLENEDITHIO)TETRASELENAFULVALENE)

Crystal structures and electronic properties of BETS compounds with te trahedral anions, MX(4) (BETS=bis(ethylenedithio)tetraselenafulvalene: M=Ga; Fe, In; X=Cl, Br) were examined. The salts can be classified in to two types (kappa, lambda). All the compounds with so-called kappa-t ype molecular arrangements retained metallic behavior down to 4 K. lam bda-(BETS)(2)GaCl4 demonstrated a a superconducting transition around 8 K. Whereas lambda-(BETS)(2)FeCl4 with the isomorphic h-type structur e exhibited a sharp metal-insulator transition at 8 K. Low-temperature X-ray studies showed that the structural difference between lambda-Ga Cl4 and lambda-FeCl4 salts is very small. The structure analysis of ka ppa-(BETS)(2)FeCl4 at 18 K indicated that the FeCl4 anion has approxim ately C-3v symmetry. The ESR measurements revealed the high-spin state of Fe3+ in lambda-FeCl4 salt and and low-spin state in kappa-FeCl4 sa lt. ESR intensities of kappa- and lambda-FeCl4 salts suggested antifer romagnetic interaction between Fe3+ ions.