3-COMPONENT ORGANIC CONDUCTORS - (BEDT-TTF)(2)MM'(SCN)(4)

New organic conductors which contain a magnetic cation of Co2+ (d(7), S=3/2), (BEDT-TTF)(2)MCo(SCN)(4) [M=K, Rb, Cs], and related materials were prepared. The crystal structure analyses clarify that alpha(BEDT- TTF)(2)MM'(SCN)(4) [M=Rb, Cs, M'=Co, Zn] are isostructural and that th ese donor arrangements are alpha-type (strictly speaking theta-type). Although the band structure calculation affords the closed Fermi surfa ce due to the large transverse overlap integrals, a metal-insulator (M -I) transition occurs at 20 K for alpha-(BEDT-TTF)(2)CsM'(SCN)(4) [M'= Co, Zn] and at 190 K for alpha-(BEDT-TTF)(2)RbCo(SCN)(4), respectively . The M-I transition temperatures of the former M=Cs salts are elevate d by applying pressure. The magnetic susceptibilities of the M'=Co sal ts obey the Curie-Weiss law with theta=3-6K from 300 K to 20K. The rel atively large theta might be originated from the superexchange of an a nion network like...Co2+...NCS- as well as interaction through itinera nt electrons in BEDT-TTF layer. Below 20 K the anomaly of chi was obse rved, which can be explained by a crystal field around Co2+.