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

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
NiCl42- and MnCl42-
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.