Crystal structure and physical properties of M=Rb and Tl salts of (BEDT-TTF)2MM′(SCN)4 [M′=Co, Zn]

Crystal structure analyses, band calculations, and electrical resistivity and magnetic susceptibility measurements have been carried out for new theta-type BEDT-TTF [bis(ethylenedithio)tetrathiafulvalene; abbreviated as ET] salts, theta-(BEDT-TTF)(2)TlCo(SCN)(4) and theta-(BEDT-TTF)(2)TlZn(SCN)(4) [abbreviated as theta-TlCo and theta-TlZn], as well as crystal structure analyses of theta-(BEDT-TTF)(2)RbM' (SCN)(4) [M' = Co, Zn; abbreviated as theta-RbM']. theta-TlCo and theta-RbM' [M' = Co, Zn] are isostructural to the previously reported theta-CsM', and the metal-insulator transition temperature increases from 20 K (theta-CsM' [M' = Co, Zn]), 190 K (theta-RbM') to 250 K (theta-TlCo) with decreasing the unit cell volume, namely, the inverse chemical pressure effect. A structural investigation for theta-RbM'[M' = Co, Zn] indicates a lattice a modulation at the metal-insulator transition temperature (190 K). For the other salt, theta-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.