Magnetic Molecular Conductors Based on BETS Molecules and Divalent Magnetic Anions [BETS = Bis(ethylenedithio)tetraselenafulvalene]

Several conducting salts based on BETS [where BETS = bis(ethylenedithio)tetraselenafulvalene] molecules and
divalent magnetic anions such as the (CoCl4)2-, (CoBr4)2-, and (MnBr4)2- were prepared. Electrocrystallization by
using the (CoCl4)2- anion gave two kinds of crystals. Block-shaped crystals were cleared to be (BETS)2CoCl4,
which is an insulator with the high-spin state of cobalt 3d spin. On the other hand, the X-ray crystal structure
analysis of a plate-shaped crystal of the (CoCl4)2- salt revealed the system to be κ-(BETS)4CoCl4(EtOH), which is
metallic down to 0.7 K. The electronic band structure calculation gave a typical two-dimensional cylindrical Fermi
surface. However, there is only very weak antiferromagnetic interaction between the S = 3/2 cobalt 3d spins because
of its anion−solvent-intermingled layer structure. On the other hand, the electrocrystallization by using the (MnBr4)2-
anion yielded the plate-shaped black crystals of the (MnBr4)2- salt. Crystal structure analysis of the (MnBr4)2- salt
showed that the salt is θ-(BETS)4MnBr4(EtOH)2 with alternating donor and anion−solvent mixed layers. The stacking
direction in one donor layer is perpendicular to those of the neighboring layers. The electrical and magnetic properties
of the θ-(BETS)4MnBr4(EtOH)2 salt showed the metallic behavior down to ∼30 K and the paramagnetism of the
high-spin manganese 3d spins. Band structure calculation of this salt gave an elliptical cylindrical Fermi surface.
Because the Fermi surfaces of the adjacent donor layers are rotated to each other by 90°, the θ-(BETS)4MnBr4(EtOH)2 salt becomes a two-dimensionally isotropic metal.