Phase transition at 320 K in a new layered organic metal conductor (BEDT-TTF)4CoBr4(C6H4Cl2)

Abstract

A new layered organic conductor θ-(BEDT-TTF)₄CoBr₄(1,2-C₆H₄Cl₂) with different resistivity behaviour that has metallic conducting layers and is semiconducting perpendicular to the layers has been prepared. A phase transition near 320 K was found in this salt by temperature variable resistivity and X-ray diffraction measurements. Precise crystal structure data, below transition (triclinic phase) and above transition (tetragonal phase), was determined. The crystal structures of both phases are composed of θ-type radical cation layers alternating with layers involving [CoBr₄]²⁻ anions and 1,2-dichlorobenzene molecules. The lower symmetry in the crystal structure of the triclinic phase is due to a non-symmetrical arrangement of the solvent molecules within the anion layers. In each structure, the neighbouring conducting layers of the unit cell differ in stacking directions. In the tetragonal phase, all conducting layers are similar in structure, and in the low-temperature triclinic phase, the neighbouring conducting layers differ in periods along the stacks (4.703 Å and 4.993 Å) and in dihedral angles θ between the BEDT-TTF radical cations of the adjacent stacks. The finding of compounds of the θ-(BEDT-TTF)₄CoBr₄(1,2-C₆H₄Cl₂) type with phase transitions above room temperature provides a wide possibility of investigating magnetoresistance, quantum oscillations, and the mechanism of interlayer transport in layered conductors.