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

Crystal structutres 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 C_3v 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.