The preparation, crystal structures, and electric and magnetic properties of (BETS)2MX4 molecular conductors (BETS = bis(ethylenedithio)tetraselenafulvalene; M = Mn2+, Fe3+, Co2+, Ni2+, Cu2+; X = Cl, Br, CN) are reported. Resistivity measurements down to 2 K reveal the coexistence of the BETS π conduction electrons and the localized magnetic moments of the anions in the κ-(BETS)2FeCl4 and κ-(BETS)4(CoCl4)(C2H3Cl3) salts. Another FeCl4 phase, λ-(BETS)2FeCl4, undergoes a sharp metal-insulator (MI) transition around 8 K. At the same temperature, a magnetic transition of the FeCl4-anions takes place cooperatively. A superconducting transition is observed at 4.6 K in λ-(BETS)2(FeCl4)0.5(GaCl4)0.5, where half of the anion sites are occupied by magnetic ions. The crystals prepared from 1,1,2-trichloroethane solutions with the NiCl4 2- and MnCl4 2- anions exhibit the behavior of a semimetal down to ≈100 K. The (BETS)4(Cu2Cl6) salt remains metallic down to 4.2 K. ESR studies show that the Fe3+ ions in κ- and λ-(BETS)2FeCl4 are in a high-spin state. The temperature dependencies of the spin susceptibilities of κ- and λ-(BETS)2FeCl4 indicate antiferromagnetic interactions between the Fe3+ ions. The crystal structure analyses of κ- and λ-(BETS)2FeCl4 have been carried out at 298 and 10 K. Closer BETS···FeCl4 contacts in λ-(BETS)2FeCl4 are observed, which is consistent with the larger Weiss temperature of this compound.

λ(BEDT-TSF)2GaCl4 has revealed to be a new organic superconductor, where BEDT-TSF is bis(ethylenedithio)tetraselena-fulvalene. The onset temperature of the superconducting transition was about 9.5 K and the mid-point transition temperature (Tc) was 8 K. The crystal belongs to triclinic system. BEDT-TSF molecules are arranged to form a “quasi-stacking structure,” where four molecules form a repeating unit.

Crystal structures and electrical properties of BETS compounds with tetrahedral anions, MX4− (BETS = bis(ethylenedithio)tetraselenafulvalene; M = Ga, Fe, In; X = Cl, Br) were examined. Every κ-type salt is metallic below 50 K. λ-(BETS)2FeCl4, which is isomorphous to the superconductor GaCl4 salt, undergoes a sharp metal-insulator transition at 8 K.

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