New BETS Conductors with Magnetic Anions (BETS = bis(ethylenedithio)tetraselenafulvalene)

The preparation, crystal structures, and electric and magnetic properties of (BETS) 2 MX 4 molecular conductors (BETS = bis(ethylenedithio)tetraselenafulvalene; M = Mn 2+, Fe 3+, Co 2+, Ni 2+, Cu 2+; 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) 2 FeCl 4 and κ-(BETS) 4 (CoCl 4 )(C 2 H 3 Cl 3 ) salts. Another FeCl 4 phase, λ-(BETS) 2 FeCl 4, undergoes a sharp metal-insulator (MI) transition around 8 K. At the same temperature, a magnetic transition of the FeCl 4 -anions takes place cooperatively. A superconducting transition is observed at 4.6 K in λ-(BETS) 2 (FeCl 4 ) 0.5 (GaCl 4 ) 0.5, where half of the anion sites are occupied by magnetic ions. The crystals prepared from 1,1,2-trichloroethane solutions with the NiCl 4 2- and MnCl 4 2- anions exhibit the behavior of a semimetal down to ≈100 K. The (BETS) 4 (Cu 2 Cl 6 ) salt remains metallic down to 4.2 K. ESR studies show that the Fe 3+ ions in κ- and λ-(BETS) 2 FeCl 4 are in a high-spin state. The temperature dependencies of the spin susceptibilities of κ- and λ-(BETS) 2 FeCl 4 indicate antiferromagnetic interactions between the Fe 3+ ions. The crystal structure analyses of κ- and λ-(BETS) 2 FeCl 4 have been carried out at 298 and 10 K. Closer BETS···FeCl 4 contacts in λ-(BETS) 2 FeCl 4 are observed, which is consistent with the larger Weiss temperature of this compound.