Three-Component Organic Conductors; (BEDT-TTF)2MM′(SCN)4

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Abstract
New organic conductors which contain a magnetic cation of Co2+ (d7, S=3/2), (BEDT-TTF)2MCo(SCN)4 [M=K, Rb, Cs], and related materials were prepared. The crystal structure analyses clarify that α-(BEDT-TTF)2MM′(SCN)4 [M=Rb, Cs, M′=Co, Zn] are isostructural and that these donor arrangements are α-type (strictly speaking θ-type). Although the band structure calculation affords the closed Fermi surface due to the large transverse overlap integrals, a metal-insulator (M-I) transition occurs at 20 K for α-(BEDT-TTF)2CsM′(SCN)4 [M′=Co, Zn] and at 190 K for α-(BEDT-TTF)2RbCo(SCN)4, respectively. The M-I transition temperatures of the former M=Cs salts are elevated by applying pressure. The magnetic susceptibilities of the M′=Co salts obey the Curie-Weiss law with θ=3–6K from 300 K to 20K. The relatively large θ might be originated from the superexchange of an anion network like… Co2+… NCS− as well as interaction through itinerant electrons in BEDT-TTF layer. Below 20 K the anomaly of χT was observed, which can be explained by a crystal field around Co2+.

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