New organic superconductor, λ-(BETS)2GaCl4 and metal-insulator transition of BETS conductor with magnetic anions (BETS=bis(ethlenedithio)tetraselenafulvalene)

Crystal structures and electronic properties of BETS compounds with tetrahedral anions, MX4 (BETS=bis(ethylenedithio)tetraselenafulvalene; MGa, Fe, In; XCl, Br) were examined. The salts can be classified into two types (κ, λ). All the compounds with so-called κ-type molecular arrangements retained metallic behavior down to 4 K. λ-(BETS)2GaCl4 demonstrated a a superconducting transition around 8 K. Whereas λ-(BETS)2FeCl4 with the isomorphic λ-type structure exhibited a sharp metal-insulator transition at 8 K. Low-temperature X-ray studies showed that the structural difference between λ-GaCl4 and λ-FeCl4 salts is very small. The structure analysis of κ-(BETS)2FeCl4 at 18 K indicated that the FeCl4 anion has approximately C3v symmetry. The ESR measurements revealed the high-spin state of Fe3+ in λ-FeCl4 salt and and low-spin state in κ-FeCl4 salt. ESR intensities of κ- and λ-FeCl4 salts suggested antiferromagnetic interaction between Fe3+ ions.