Phase transformations of iodine-containing dual-layered conductor θ-(ET)4CdI4(PhCl) upon cooling

Abstract

A layered organic conductor θ-(ET)₄CdI₄(PhCl) has been synthesized and characterized (ET is bis(ethylenedithio)tetrathia-fulvalene). It belongs to the family of dual-layered conductors θ-(ET)₄MBr₄(C₆H_6−nX_n) and for the first time includes the iodine-containing anion [CdI₄]²⁻. It exhibits metallic conductivity within the plane of organic ET molecules down to 206 K, and semiconductor properties in the perpendicular direction. The temperature dependence of in-plane resistivity for the compound shows changes in the conductive properties at 199 K and below 164 K. Structural phase transitions have been revealed at these temperatures. Crystal structures determined at 220, 185, and 100 K are attributed to tetragonal, monoclinic, and triclinic crystal systems, respectively. As a result of the structural phase transition at 199 K, equivalent ET layers become nonequivalent, the dihedral angles between ET molecules of neighboring stacks change, and ordering of terminal ethylene groups of ET molecules occurs in a half of radical cation layers. At 100 K, the difference between neighboring radical cation layers increases as compared with the monoclinic phase and the ordering of terminal ethylene groups in all ET molecules has been found. The solvent molecule PhCl remains disordered in all these structures.