Crystal Structure and Physical Properties of M = Rb and Tl Salts of (BEDT-TTF)2MM′(SCN)4 [M′ = Co, Zn]

Abstract

Crystal structure analyses, band calculations, and electrical resistivity and magnetic susceptibility measurements have been carried out for new θ-type BEDT-TTF [bis(ethylenedithio)tetrathiafulvalene; abbreviated as ET] salts, θ-(BEDT-TTF)2TlCo(SCN)4 and θ-(BEDT-TTF)2TlZn(SCN)4 [abbreviated as θ-TlCo and θ-TlZn], as well as crystal structure analyses of θ-(BEDT-TTF)2RbM′(SCN)4 [M′ = Co, Zn; abbreviated as θ-RbM′]. θ-TlCo and θ-RbM′ [M′ = Co, Zn] are isostructural to the previously reported θ-CsM′, and the metal-insulator transition temperature increases from 20 K (θ-CsM′ [M′ = Co, Zn]), 190 K (θ-RbM′) to 250 K (θ-TlCo) with decreasing the unit cell volume, namely, the inverse chemical pressure effect. A structural investigation for θ-RbM′ [M′ = Co, Zn] indicates a lattice a modulation at the metal-insulator transition temperature (190 K). For the other salt, θ-TlZn, a semiconductor–semiconductor transition has been observed at 165 K, where the behavior of the magnetic susceptibility, following the two-dimensional Heisenberg model, changes from J = −30 K to −57 K under a slow cooling condition.